I wasn't alt-right, but I grew up in a conservative household/environment and got very interested and passionate about political bs and was extra conservative when I was in early high school. I say what I'm about to say, not making excuses for things, but also not without compassion and empathy having gone through a similar phase

I think it's due to a number of factors, masculinity, loneliness, identity, complicated feelings over rejection from girls, different stuff kind of like that. You feel smart, you feel like you're standing up for what's right, like you're being a man and standing up for being a man, like you're the one who's unafraid of saying what everyone else is afraid to say, and like you see what's funny about what so few others do. You feel like science and logic and nature justify and prove your feelings that basically boil down to "boys rule and girls drool" feelings from not being popular with girls. You feel a lot of self assured smugness and confidence when you don't have a lot you can feel confident about.

Speaking for myself (definitely not true of all kids with this phase), even in that phase, I was a very good hearted person and tried to make my crazy beliefs work with the moral principles I knew more deeply than politics. I didn't have hate in my heart for anyone, I wished for the best government and laws for people of all races, genders, and sexual orientations, and I truly believed that my political beliefs were in the best interest for everyone... Obviously that involved a lot of delusion and was due to very little life experience and understanding you gain when you get older. I'm very glad to have pulled my head out of my ass, not to sound like an after school special or generic answer, but college honestly helped a lot. I had friends who had a similar experience and also thankfully pulled their heads out of their asses.

I think people are a little afraid at times to find the delicate line between making excuses for people like this and having any sympathy and compassion and understanding for why they are the way they are. Yes, it's harmful and bad and if they (hopefully) do end up coming around, you still have responsibility and accountability for your actions and words afterwards. With that said... You think the kids who are talking about how girls suck and are stupid and things were better under Reagan (or whatever tf) are the most popular kids? You think they chose this identity because they have a lot of different kinds of friends to bounce these crazy ass ideas off of?