>>1
To be a professional hitman you need 3 things.
1: To be unidentifiable. If you aren't on police records a skin flake or a fingerprint won't put you in jail for life, if you are, go to another country and don't let anyone know you have gone. Use a silenced pistol, one shot to the head ata distance, or inject an air bubble into their neck while they are asleep, whatever method of killing is the perfect crimes etc... Always completely cover your body with clean clothing and burn everything every hit. Plastic overalls are best for this. If you need a weapon, or various tools you can buy anything over the black market without leaving a trace, just make sure the dealer has a reputation. If you have to stay somewhere, make sure there is a large distance and no one sees you between your accomodation and your hit, security cameras, cats, no one.
2: Money. This is simple but difficult, get someone to pay you to kill people. You obviously want the person to pay you and not arrest you so you need to ensure they are not police, which rules out meeting the contractor in person. Using the phone means you leave a trace which risks #1. This means you will have to kill for an organisation that will need your services in the future and you must maintain contact with them through anonymous letters (dont lick the stamp). You also need to recon your victims to make sure it isn't an elaborate police sting.
3: Tactics. The higher your athletic ability the more avenues to arrange hits and kill a person with minimal risk there are. The more intelligent you are the more factors you can take into account, the further you can reason things through and the more avenues to arrange hits and kill a person with minimal risk there are.