I have no problem with the Rangers targetting Karlsson, so long as they do it legally, or they get called for illegal hits.
I know it's very popular to call late hits "finishing your check" but I believe it is bull$417 when it is more than a split second. The only legal reason for a body check in hockey is to separate the player from the puck, so when the player is not in possession of the puck there is no reason not to let up. I realize the game is played at a very fast pace, but even the pluggiest of the plugs in this league can stop on a dime or veer off. I understand a split-second, but when they keep coming in 1 second or more *after* the puck has been cleared out -- usually with hands/elbows/sticks up high -- that needs to be called for what it is -- a blatant attempt to injure or at lease intimidate.
If the kid is good enough to get the puck out of the way before you get there, say "Well played" and try to be quicker/faster/stealthier next time.
If you are too slow to deliver a hit legally, then rest up for 2 minutes and try again.