If a person believing suicide to be halaal, kills himself, will he be a kaafir?

Answer

Believing any haraam act to be halaal, not only suicide, is kufr. If someone believes that suicide is halaal, then he is in the same category as the person who claims that fornication is halaal, even if he does not commit suicide and zina. In both cases he loses his Imaan. If he does not return to the fold, he will dwell in Hell forever.