So I am playing a halfling rogue assassin, that has been blessed/cursed by Shar and must offer up a blood sacrifice in her name periodically.

Our team was in a cave and we were attacked by goblins, after finishing all but one off, my character began extracting information from the goblin. After we were done my character asked if he was in pain, the goblin replied yes. So my character went to end the creature's suffering.

However at this time another PC attacked me with his mace and I backed off, not wanting to pull the party off story. Problem is, my character is an assassin, is known to have an obsessive condition, when it comes to targets.

The PC that attacked me kept moving the goblin along with him and I just waited. Then I had an opening, the PC became distracted with possible loot and left the goblin unattended. I struck, rolled a natural 20, and cut it's head clean off. I then tossed the head into the room remarking on where his friend is now.

This of course sent the other PC into a rage Abe he charged me, another party member grappled him, but he lashed out and knocked that character unconscious.

I feel that I could kill this character, but decided to pull back and stealth away.

The player, not just the character seemed to be genuinely upset outside of game.

Am I wrong for playing my character this way?