Being Human Vampirism and Mitchell
This contains specific spoilers about season one, and very generalised potential spoilers from season two (Mitchell is taken from the end of S1 but we learn more about how vampirism works and his backstory in S2).
The very original plan for the show that became Being Human was three 20-somethings sharing a flat. Since Toby Whithouse thought that sounded like the most boring idea in the world, he decided to give them all interesting flaws to deal with. One would have very low self esteem, one would have anger management issues, and one would be a recovering sex addict. Then he realised that he could map that onto a ghost, a werewolf and a vampire, and so Being Human was born.
This is important because even though that original plan was mostly scrapped, the sex addict thing still forms something of a basis for Mitchell and for vampires in Being Human in general. Drinking blood is firmly linked to sex- Mitchell refuses to have sex in S1 because he doesn't feel like he can stop himself from drinking from his partner if he does. Vampires who drink a lot of blood act differently to vampires who don't. They're much more overtly sexual and mostly don't seem to have much self-restraint in many other directions either. How much varies based on their personality, but the effect is always there (in S2 we see a very clear example of the difference between a vampire's personality when high on blood in constrast to when not). Mitchell himself is a pretty tactile person even when abstaining, and doesn't have the greatest self control either.
This personality change and lack of inhibitions acts as a shield against the knowledge that they're killing hundreds of humans. Mitchell talks about blood as an addiction, but at one point explains that it's not really a typical physical addiction. It's just that if a vampire stops drinking blood they start to remember all of their victims and they start to feel the guilt that a normal person would feel at having killed those people. This is what they have instead of withdrawal, and most vampires don't make it past that stage without being overwhelmed and going back to blood just to make it stop. The similarity to addiction is that even after they've survived past that stage, there is always a possibility that they will begin to feel blood lust again and they might not be able to control it. In S2 a vampire survives 20 years without blood before losing to it, but he does lose in the end.
There's also immense social pressure on them to drink. Vampire society is proud of itself and proud of being predators towards humanity. This is something that Mitchell constantly struggles with. He was basically a rock star amongst vampires for well over 50 years before he seriously thought about trying to stop drinking. He was Herrick's closest friend and partner in crime (I assume they had sex but only really as a side thing, related to them drinking blood together. He doesn't think of Herrick as a lover at all) for a very, very long time. Even believing that human lives should be respected, he is still very proud of his people. He reminisces fondly about his days as a killer until he remembers that he shouldn't. It's difficult for him to dissociate himself from them. After all, he's not human and he can't fit in with humanity, so if he doesn't fit with the other vampires then he doesn't have a group to identify with. And that's an incredibly difficult way to live. His compassion for humanity is balanced on a knife edge- it's very, very easy for him to slip into his old ways of thinking. We see this happen in episode 4 of season one, for example. All it takes is one example of how humanity can be arseholes and he's willing to abandon them and come back to the safety and acceptance of the other vampires. He might try and see humans as people, but he has a tendency to take individual incidents and paint all of humanity with the same brush because of them.
The very original plan for the show that became Being Human was three 20-somethings sharing a flat. Since Toby Whithouse thought that sounded like the most boring idea in the world, he decided to give them all interesting flaws to deal with. One would have very low self esteem, one would have anger management issues, and one would be a recovering sex addict. Then he realised that he could map that onto a ghost, a werewolf and a vampire, and so Being Human was born.
This is important because even though that original plan was mostly scrapped, the sex addict thing still forms something of a basis for Mitchell and for vampires in Being Human in general. Drinking blood is firmly linked to sex- Mitchell refuses to have sex in S1 because he doesn't feel like he can stop himself from drinking from his partner if he does. Vampires who drink a lot of blood act differently to vampires who don't. They're much more overtly sexual and mostly don't seem to have much self-restraint in many other directions either. How much varies based on their personality, but the effect is always there (in S2 we see a very clear example of the difference between a vampire's personality when high on blood in constrast to when not). Mitchell himself is a pretty tactile person even when abstaining, and doesn't have the greatest self control either.
This personality change and lack of inhibitions acts as a shield against the knowledge that they're killing hundreds of humans. Mitchell talks about blood as an addiction, but at one point explains that it's not really a typical physical addiction. It's just that if a vampire stops drinking blood they start to remember all of their victims and they start to feel the guilt that a normal person would feel at having killed those people. This is what they have instead of withdrawal, and most vampires don't make it past that stage without being overwhelmed and going back to blood just to make it stop. The similarity to addiction is that even after they've survived past that stage, there is always a possibility that they will begin to feel blood lust again and they might not be able to control it. In S2 a vampire survives 20 years without blood before losing to it, but he does lose in the end.
There's also immense social pressure on them to drink. Vampire society is proud of itself and proud of being predators towards humanity. This is something that Mitchell constantly struggles with. He was basically a rock star amongst vampires for well over 50 years before he seriously thought about trying to stop drinking. He was Herrick's closest friend and partner in crime (I assume they had sex but only really as a side thing, related to them drinking blood together. He doesn't think of Herrick as a lover at all) for a very, very long time. Even believing that human lives should be respected, he is still very proud of his people. He reminisces fondly about his days as a killer until he remembers that he shouldn't. It's difficult for him to dissociate himself from them. After all, he's not human and he can't fit in with humanity, so if he doesn't fit with the other vampires then he doesn't have a group to identify with. And that's an incredibly difficult way to live. His compassion for humanity is balanced on a knife edge- it's very, very easy for him to slip into his old ways of thinking. We see this happen in episode 4 of season one, for example. All it takes is one example of how humanity can be arseholes and he's willing to abandon them and come back to the safety and acceptance of the other vampires. He might try and see humans as people, but he has a tendency to take individual incidents and paint all of humanity with the same brush because of them.