Is a brother who said he would pay someone to kill him, and then was found dead, considered to have committed suicide?

There is not enough evidence to accuse your brother of suicide, and one should think well of him, for death comes suddenly. If it is assumed that he committed suicide due to a mental state he could not control, then he is not to be held accountable for that. One should avoid discussing this topic with people to prevent ill-conjecture about him without proof. Allah Almighty said: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." Suicide is a terrible act with severe consequences, and Allah and His Messenger (peace be upon him) have forbidden it.