My question is if a person asks for help, he needs financial help and is giving the reference that he is collecting funds in order to help poor but in the long run I know he won't use it for that. He may use it to kill some people or something like that but it is my assumption and moreover is mostly expected. What should I do. Should I donate or not?

If you just think or assume he is abusing money, but you have no proof of that, you are not allowed to tarnish his image among others.
Donating from your side is not a must, and it depends on your confidence.