No, he's not even doing that. At the absolute best, he's telling me how he perceived my character commit one action. If a mother kills something threatening their child, some could see the mother as evil, others not. Saving one's own life under duress is not necessarily evil.
Even it if is, that's one action, regardless of anything else how the character had been played. Considering that the ogininally posting DM had different expectations of what his first blush of how the paladin would respond, we can pretty well assume that the character had not been played evil.
Alignment in 5e is solely an RP tool - there is no mechanical connection, no way to detect it, nothing it interacts with within the rules. It does not affect reputation, give an "alignment language" (for us Grognards about), or have anything to do with how people in the world see you. It's listed in the personality and background section of the PHB - it's an RP tool.
So let me correct my statement. At best, the DM has decided to take one action under duress that likely does not match with how the character is generally played, deciding how they perceive it is the only possible interpretation, and then act on that to change an roleplaying-only guide on my character sheet. Since it has no affect except RP within the rules, there really is no assumption besides he expects it to affect my RP unless you want to posit that it's just a power trip that is not supposed to have any meaning whatsoever.