In the old days, they were called hatchet men and their job was to retaliate against the other teams. Sometimes they were called goons.
In the 60's we had a reputation for it. Red once hit the owner of the St. Louis Hawks, Bill Kerner in the jaw over the height of the rim. Their forward, Alex Hannum called us
"hatchet men". I read somewhere that Tommy would grab a guy on a charge to make it look good and pull him down on top of him. In 70's we had Dave Cowens who once elbowed Mike Newlin in the face to illustrate what a foul was to a ref and told him "Now that is a xxxxing foul." in the 80's we had McHale clotheslining Rambus. Out of the 50 dirtiest NBA players ever, Dave Cowens, Kendrick Perkins, Paul Pierce, Shaquille O'Neal, Xavier McDaniel, Danny Ainge, Kevin Garnett, Kevin McHale and Bruce Bowen all made a list by Bleecher Reports as the 50 Dirtiest players of all time. That is almost 1/5 or 18% of the dirtiest players were Celtics. We've always had a rep for it, folks. I don't live in Boston nor have I ever but Celtics have always got the shaft in this regard.
http://bleacherreport.com/articles/530978-the-50-dirtiest-players-in-nba-history#page/51In any sport there are guys who will jack you up if they get a chance. Big guys love knocking people senseless on picks when they can. It happens all the time and yes its people trying to hurt people. Not kill them mind you, but rack them up but its a freeshot to use your bulk/size on someone. It's not done as well as it used to be in the game because peeps really do not know how to set a good pick. Should be shoulder to shoulder with the pickee and the picker is not supposed to be moving when he sets the screen. But you can hit a defender as hard as you like as long as its his momentum providing the impetus, legally.
Guys also push peeps and elbow routinely all the time. It's a reality of the low post. Your elbows are your protection and early warning system to tell the other guys to back down. Both are fouls and should be. But it happens all the time.
Certain contact like holding off a defender when driving or dribbling happens routinely. Its not designed to hurt peeps but get an advantage. It should be a foul under strict interpretation of the rules. Jordan did this all the time.
That being said the unwritten rule is that you don't submarine people or undercut them when they are in the air. Picks are fair game as they happen the flow of the game. Its the actions outside the game flow that peeps get mad about basketball. It's never ok to slide under a guy.
But basketball has always had a physical element. KG doesn't even play that physical really folks. But its called PF which means Power Forward not Prissy Forward. What KG did was not even that bad folks.