You don't know how it effects someone, in cases with national tragedies, I'd say if i think someone will be upset by it:
DONT POST IT
That goes both ways, I'm sure he is probably posting in response to your post because it probably bothered or upset him. I'm sure we all like to be reminded the people that died that day. You just bothered someone with your post, is that going to make you "NOT POST IT" anymore? or are you free to express yourself. At least this person was nice enough to put it in a spoiler tag, no one is forcing you to open and look~