If you remember, before Vito Corleone, it was Don Fanucci who was ruling the neighbourhood. I think he jumped from a small burglar gang with his friends to organized crime when he didn't like how Fanucci exercized his power and took him out.
You can see in this video how Fanucci was too heavily into extortion and was responsible for Vito being fired. This might have induced personal resentment from Vito.
When he was in charge of the neighbourhood, he was more protective of the other people, hence more respected.
Why did he murder Don Fanucci?
I suppose when you ask "Why cold-blooded murderer?", you are talking about Don Fanucci. This question explains it well. As Don Fanucci was not liked by the people, as he was putting too much pressure on them, and Vito supposed he was not as powerful as he seemed, he took his chance to take him out. He had probably in mind that he would be more careful to his neighbours.
It is the opposite. The Genco Olive Oil Company was created after he started illegal businesses to cover illegal incomes. You can see here, when Roberto the landlord comes to visit Don Vito to apologize and return the money, Vito's office being decorated as Genco Import Co. meaning that he was already established as the local godfather:
The Godfather wikia page says:
The Company was a front for the family's criminal activities.