Is human nature fundamentally selfish or altruistic?

If you mean selfish in the sense wanting to fulfil own desires by harming others others it No. This is not human nature it’s acquired with situations or past experiences. Basically you can say it’s acquired. Characteristics.

I wouldn’t say it’s a choice they do knowing it’s unconscious choice.They just so pleased with themselves they just think the whole world is revolves around them or they are unaware of the side effects around them.

Basically. They are not aware of the side effects. Of their actions.

Most of the people learn it with experiences to change with experience and time