I think it would be so fun to serve milk to my lactose intolerant roommate.
I wouldn't wish this on my enemy. Why would you do something so terrible as giving them milk?
I'm going to ruin someone's life for no reason
That could kill them, or at least really harm them. It would be better to give them an alternate drink that they can enjoy.
I'm going to do it anyway.
That is a very mean thing to do and possibly illegal. You should not do this and get a new roommate.