Lots of definitions, but when a baseball pitcher is referred to as a headhunter it means that he intentionally throws pitches at the heads of batters on a regular basis. This is done either to intimidate them or to actually crack their skull.
Pitchers who do this are under the delusion that they have the right to mete out punishment to batters for such minor offenses as acting a little too cocky, or swinging at a 3-2 pitch when the batter's team is leading by 10 runs. Sometimes they do it just because they are frustrated that the batter gets a lot of hits off them. They do not deserve to be in the major leagues. They do deserve to have their asses kicked by 'roided up hitters who charge the mound after being thrown at.
"Shit, Roger Clemens hit Mike Piazza in the head with that pitch! And the last three times Piazza faced Clemens, he homered off him! I bet Clemens did it on purpose; that prick's a headhunter!"