How many people have you observed: tens, hundreds, thousands, or tens of thousands? How long have you observed them: minutes, hours, days, weeks, years or decades? The random and casual observations of people you come across online and offline can only tell you so much and why they behave the way they do. If we really want to know how negative behavior like this develops we have to dig much deeper into their entire lives and not assume that it's caused by a few factors like incel sites, violent video games, social media, women, etc.
If we really want to know he could do such a heinous act like this happen then we must also ask how other people treated him in school, his brief stint in the army, etc. It's been reported he has Aspergers and was in special needs classes. I would expect that people avoided him because he was "pathetic" as you say and likely had few friends. What happens when people are shunned or isolated like that for years or decades? That feeling of isolation coupled with his disorder, the rigid structure of the military and many other things all contributed to how he ended up deciding to do this heinous act.
I think the root cause of negative behaviors like this come from how we treat people that are different from us. The more we view people that are different from us a "losers", "pathetic", etc the more we isolate them. The more we isolate people the more we cause groups like incels to form and drive people like Aleck right into their arms.
I also think that part of the remedy to these issues are to try to include people that seem isolated (sitting alone at lunch, awkward, quiet etc.) into your social circle. You come across people like this all the time at school, work, events, etc. It's obviously not a magic bullet or a one shot deal that will solve everything, but it should certainly help. More importantly, it's a decent and kind thing to do for a fellow human being. We definitely need to teach our kids to do the same thing.
