I hear you, that you were trying to help and instead got caught in this negative spiral of negativity from this person. Sometimes we need to take a step back and sometimes we need to actively get involved. There are risks either way and people will judge us either way. Your intentions were noble and hopefully that will be recognised on the long term.
It seems this person is being toxic and getting off on spamming. I think if you keep downvoting him or responding to him, he will continue. He is acting like any other toxic person (or narc or abuser). Don't feed him, don't be his supply. It might be best for you to ignore him. Perhaps report him to steemit-abuse, and then take a step back and ignore this person so that they stop feeding on you. Your energy is best kept for others who will appreciate it and not take it to fuel their negativity.
Agreed. In fact their behavior makes me wonder if they're not the original spammer/advertiser. Especially, as someone else pointed out, they created that account in December and only now just started commenting.
Either way though -- fighting with a tar baby tends to make one dirty, you're right.