Honestly?
If you don't love the dysfunctional 8-year-old and can't imagine co-parenting when he's with you, then this relationship is not for you.
Harsh? Probably. But also honest. Because a kid who comes from an alcoholic home needs security and love, and if you can't provide that, you won't be the step parent he needs. He needs one healthy home, and if that's beyond what you can manage for him on breaks, break it off with his dad. Because no matter how much you try, if your heart isn't in it, if you see him as a burden -- trust me, he will feel it.
I'm saying that coming from a situation where I have offered my husband several times to separate because I feel like loading my kids-with-baggage on him is more than I could possibly ask of any human being. He says "the kids are part of you, and I want all of you." And he stays.
I don't see you as a bad person for questioning whether you can handle this 8-year-old. I see my husband as friggin Superman. If the tables had been turned, if he had come with kids with as many problems as the ones mine have -- I would have loved having a relationship with him without his children. And of course, he wouldn't have wanted a relationship with a woman who couldn't handle his kids.
I see how incredibly needy and hurt my children are, and how nothing is ever enough for them. They are like little black holes of neediness, and they're everything from just vile to ignoring their stepdad at times. I cringe every time. And it also affects my self-confidence. Because I think "no way am I worth taking all that crap for"...
I think it's a perfectly sane question to ask -- do I have the capacity to take on this hurt and harmed child? -- and it's perfectly OK to answer the question with a NO. I guess the question I would ask myself is "Is the relationship with this man worth my really honestly working hard at helping out his kids during the time he is with us?"