When the question of is 'hiring an assassin to kill someone evil' is asked, the critical question is not 'am I doing evil to the person I kill?' because that has to be adressed separately when we ask 'is killing someone evil'. The critical question is 'Am I doing evil to the person I hire to do the killing?'. And, I believe you are.
For the person to be an assassin implies that the person is an assassin professionally, and this profession is by its normal interpretation one which is evil. If you are a 'hitman', then you take jobs killing people and you don't by your professional code take much consideration into who you kill just so long as the money is there. By hiring an assassine, you are either encouraging the continuation of this profession, or else encouraging someone to take up this lifestyle professionally. Either case is an act of evil, regardless of how worthy your particular target is of death in your opinion.
Now, we could of course modify the terms of the expression somewhat. If you are a duly appointed official of a sovereign state who has been intrusted to protect the group, and the assassine is a duly appointed official who has also been intrusted with protecting the group, and the target is a target which magistrates have declared to be guilty of a crime which involves murder of the citizens of that state or else intent to murder same, then the 'assassination' isn't to my mind of the same category of behaviors. In this case, the assassination is an act of war and could be legitimate self-defence and must be judged good or evil according to the ways that we judge wars just or unjust. However, because of the difficulty in judging whether or not someone is worthy of death we DO NOT normally leave this matter up to an individual and even groups of thoughtful people with what appears to be clear evidence can go wrong - so it is a measure to be taken with extreme care if at all.
I cannot think of a case were a private individual would be justified in hiring an assassine. Even if he has clear need of killing someone and the authorities cannot be trusted to perform the act (perhaps because they are evil themselves), the act of hiring when death is on the line taints the event. If someone needs to die, the person doing the killing must do so from the conviction that what he is doing is just and necessary, and not from the motivation of greed or even professionality. Soldiers should kill because they believe it necessary to defend thier loved ones, and not because it is thier job and thier boss ordered it. No soldier can defend a murder by saying 'I was ordered to do it.'
How much less can you defend it with 'I was paid to do it.'
And, the smaller the group deciding who must live and die, and the more secretly this is done, the more skeptical you must be of your own decision making process. You and a professional killer do not constitute reasonable jurisprudence.