I do feel bad for the amateurs. I went to art school where we received and administered constant daily critique where frankness matters. Genuinely mean spirited comments obviously still sucked, but we couldn't hesitate to say things like "that nose reads more like a foot and that flesh kind of makes them look dead, so unless you were going for that, maybe you could consider [etc]" because class was only 4 hours long, 15 people needed crit, we still had another huge drawing to complete, and after you've been staring at a piece for hours or days, you can't even see it objectively anymore, so you're thankful for the reality check. It was technical stuff-- not commenting on people's ideas or what they were trying to express. That experience moves "the line" we instinctively don't cross as social creatures, and something we might say with the best intentions without reading the room could entirely put someone off of learning art, forever. Even if it seems constructive you're saying it, if it's received as mean spirited and is out-of-step with the tone of the exchange, then the intent doesn't matter much.