Whispering Someone to Tell Them How Bad They Are

I cannot understand why people do this, so I am going ask the community.

My current favorite alt is a Holy Priest, GS 435. I have a full 5 piece set but a couple 8/8 champion pieces that I haven't gotten upgrades for. The toon is AOTC and I've gone through Heroic Aberrus 10-15 times, even more reps on common bosses before PUGS fall apart.

I was healing a H Aberrus today. I joined at Assault and we one-shot every boss up through Echo. We were going pretty good but numbers were all over the place. Some DPS was over 100k and some were down in the 30s and 40s. Heals were basically the same way. I was around 60-90k HPS depending on the boss - one player was always over 100k and the lowest was 40s-60s. No one really cared because we were one-shotting and even if newer players died early to stupid things, we were doing fine and didn't wipe. I did 65k HPS on Echo and was #3 of 5 for the healers. #1 was 110k and #2 was 90k. #'s 4 and 5 were both in the 40s and 50s.

After we finished Echo, we cleared the trash down to Sark. Once we were all on the platform, the RL did an announcement that said "Thanks, reforming for Sark" and everyone was kicked. A few folks said "WTF" and "way to get carried" out loud and I said it was BS and that the team was doing fine. Everyone hearthed out and 4 seconds after I got back to Valdrakken, I got a whisper from the #2 healer and he said "You were only 65k on Echo" and then "ur literal garbage". I tried to whisper back but it said no known player found (which seems to happen every time I try and reply to these whispers). I've gotten whispered like this a few times in the past, sometimes someone is cranky about a PvP battleground and sometimes after a raid falls apart.

I simply don't understand. I have never, in all my years of playing, felt it necessary to whisper another player to tell them they suck. Sure, I've called out low DPS in M+ in part chat, especially if they are doing dumb things that kill the party, but I've never singled someone out and whispered for no better reason than to tell them they are bad.

WHY DO PEOPLE DO THIS? What did that player get out of it? And I was 3 of 5 healers. 65k HPS isn't amazing but I've seen way, way worse and no matter what, we one shot all the bosses. Sure we may have wiped on Sark, but last time I did H Sark I pulled 92k HPS. Did he whisper the other 2 to tell them they were garbage or was it because I complained that we carried the RL to Sark and then we all got dropped, which was a totally BS thing to do regardless.

Please explain this to me.

Edit: gear score