Murdering a murder is committing evil to punish evil. In that it would be evil/good. It's still evil, but also good that something that is only evil got erased. Killing would still be evil. War is evil. But apply the above, one could be good and evil where one is usually only evil. For instance, if the war is to topple a ruler that is only evil... kills, imprisons, rapes, etc. Then something good is committing evil to stop that pure evil. The soldiers that fight for that pure evil would need to be evaluated individually. Why are they personally fighting for that evil? If it is so they don't get killed by said ruler then they are also committing evil for a good... to save their own lives. But they are still committing evil when they are killing another. There is also degrees of this and i think the degrees can be best described by forgiveness or thoughts of forgiveness. If one learns the solider's family would have been killed if he didn't fight... then even if he/she cannot be forgiven... a sense of understanding or a thought of forgiveness could follow. Pure evil however cannot be forgiven or it would be extremely very hard to forgive.
Is any of this objective? I don't know if i could truly say that with certainty. We are the ones defining all this which would always point towards subjectivity. However, i think we would mostly always define the above mentioned evils as evil. Even someone that is evil would think it's evil if someone murdered them (well they wouldn't know), or did some form of imprisonment or rape. There could be degrees of it... maybe someone wants to be raped, in which case that would turn it good for them, but stay as evil for the one committing it. (*Edit: I just noticed this last example is tricky. The one committing it technically wouldn't be committing evil if the person wanted to be raped. It's an example of the evil/good in that case.)