I’m not going to go into the nature of good and evil, or the true causes of evil here. That’s a long topic, and there’s a lot of other material about it on this site.
But the oversimplification implied in the question is dangerous, and all too commonly believed.
Let’s consider some Human motivations.
You should know the source, but the list is good regardless of source.
Pride
Anger / wrath
Sloth / laziness / greed for rest.
Gluttony - greed for food.
Lust - greed for sex
Avarice - greed for economic wealth / power
Envy
One can add greed for power. Greed for attention. Greed for fame. Greed for acclaim, recognition, etc.
Power comes in many forms - economic power, social influence, political influence, military power, physical power etc.
People also desire security, health, beauty, pleasurable experiences etc. etc. Security ties in with justice. People want protection from injustice, unless they are the ones being unjust.
If you define selfishness as being the desire to satisfy all of these desires, then you could say selfishness is the root cause of bad human behaviour.
But it’s not really true.
A troll isn’t being selfish. They pull others down but get little out of it except a sense of power, a stab of envy satisfaction, a moment of anger resolved by trashing someone else.
Someone drowning who pushes others down to lift themselves up is selfishness. Someone on dry land who enjoys hitting others in the sea over the head with an oar until they drown isn’t being selfish.
Unfortunately there is a lot more to human ‘evil’ and ‘bad behaviour’ than merely being selfish. Stalin didn’t send millions to their deaths by starvation and the gulag because he was selfish. Hitler didn’t kill Jews because he was selfish.
A deeper understanding of humanity, its motivations, and its social structures is required to understand its ‘bad’ behaviours and the nature of evil.
Terry Pratchett had one of his wiser characters - a witch, declare that evil begins when people are treated as things.
Stalin said, “One death is a tragedy. A million deaths is a statistic.”
This has very little to do with selfishness.