The literal definition of a cuckold is a man who was cheated on. If you feel he made up for it by killing the dude, that's fine, but it's beside the point.
The title stands.
Incorrect, being cheated on does not make someone a cuck.
A cuck is someone who is too weak to stop it, or someone who keeps allowing it to happen.
A cuckold is also a male which unwittingly invests parental effort in juveniles that are not genetically his offspring.
The dude took matters in his own hands when hearing about the lovers affair, that is something a real man would do, the opposite of what a cuck would do.