After defeating and interrogating a bound redbrand ruffian in a dungeon, I decided to kill him after he cannot provide more information in order to prevent him from being freed later on (or potentially freeing himself) to become a threat again as we were not planning on leaving the enemy infested dungeon anytime soon.
My GM immediately says that this is an objectively evil action as I had complete control of the situation and was not immediately forced to kill to survive. Furthermore, he said you lied saying you'd let the ruffian go if he cooperated in an effort to gain intel from the ruffian. He then said if I were to continue killing defeated and bound enemies, I will have to switch from Chaotic neutral to Chaotic evil.
I however disagree, as from my understanding Chaotic neutral simply tries to maximize their own chances of survival at minimum expense to themselves and innocents. I believed that since the ruffian was an aggressor attacking the party and others, he was definitely not innocent and therefore fair game. The lying and killing were both in an effort to maximize survivability as it gave valuable intel and removed a potential future threat. The DM said that I should have thought about other options like carrying him out of the dungeon or simply left him there tied to a pole. That, however, would've meant lugging around a deadweight that could at any time shank me in the back, or as mentioned before, become a threat again by being freed by his friends or himself.
So my question is, does not taking every possible step to avoid killing mean that the action is evil? Or so long as the character is doing it for self-interest it is good enough to justify neutrality?