Legit don't get that criticism tbh. His upbringing is what made him Killmonger. He grew up in the hood around so much violence that he became de-sensitized to it. The only person that he cared about was his father, who told him about his heritage. After King T'Chaka came and confronted his brother/ Killmonger's father and his father betrayed and killed, Killmonger grows up with a chip on his shoulder and wanting revenge.
He uses the military to get out, becomes a elite soldier and uses his fathers notes to link up with his former contact, Klaw, who was smuggling Vibranium and weapons out of Wakanda. His father was planning to use the weapons to help overthrow the ruling class and help the oppressed, and is what Killmonger's motivation is, along with ing over Wakanda-- who abandoned him and left him to fend for himself. He pretty much says that he took the training that he got from the U.S military just so he can use it against them.
As far as character motivations go for a villian, I think its pretty great tbh. Had Wakanda not been so isolationist, his father wouldn't have needed to work with Klaw to smuggle weapons out and disobeying the rules. Had T'Chaka taken him back with him, he wouldn't have turned into Killmonger, had he not grown up in Oakland he wouldn't have turned into Killmonger either.
It can be argued that his personality was a fascade, not showing weakness. He legit cries in the ancestral plain after talking with his father, and after being beaten by his cousin, he shows humility and even respect towards T'Challa after realizing that his mission had failed. He cries as he watches the sunset, because it reminds him of his father. He made such an impact on T'Challa that he reconsiders Wakanda's role in the world and decides to share their tech and knowledge with the rest of the world.
He's the most fascinating part of the movie tbh.