What inspired Roland's assassin persona and supernatural abilities?

The Dishonored franchise.

Is Roland evil or a psychopath?

Absolutely not. While he may appear to be so, he must abide by a set of rules set forth by the religion and the Outcast where his powers were granted from.

Who is the Outcast?

The main deity who allegedly created the Corvidae species and distributed supernatural abilities to ensure their survival and continuation. The Outcast himself is inspired by the Outsider from Dishonored.

Roland appears to have a lot of intimate moments. Doesn't this break his main character and violate The Scriptures?

In his canon universe, he has little to no sexual encounters as it affects his mana and makes him vulnerable. However, a non-canon version of him exists in which he can do whatever he pleases. The naughty stuff is there to please others, and myself of course.

Is Roland also bisexual/gay?

99.99% of the time he engages in straight relationships, but there are rare instances of him getting dominated by other male characters either forcefully or just out of curiosity. Again, none of this is canon.

Why did Roland become an assassin?

His abilities provide a great suite of tools for this kind of work that makes him virtually unstoppable. So he chose to become an assassin for hire in the modern world where he receives a handsome source of income.

Who are his clients?

Usually rival criminal gangs, wealthy individuals, politicians, and even some government entities.

Does he have a lot of friends in his canon universe?

He does have some, but not a whole lot. None of them would know of his occupation.