Homocidal thoughts?? I'm 14?

I'm only 14 and I've been having homocidal thoughts for probably 2 years now. I have a strong urge to kill a human being. But, I'm obviously not going to. I don't have anger issues, I'm actually a very nice kid who gets good grades at school. The only thing that may have influenced me to think these is that I have a very alcoholic father whom I really dislike. And I would never hurt any animal, I grew up with pets. That's what confuses me. These urges hit me at random times and I can't stop thinking about them. And when I tell my parents or older sister, they laugh it off and think I'm joking. I'm started to get scared and worried because these urges are becoming stronger and I'm starting to "accept" them instead of fixing it. What should I do to prevent something I'll regret later on?