Yes, mercy is a good quality in a human. You are willing to scar your own soul so another living thing does not have to suffer. The animal would have died in a few hours and would have gone through a lot of suffering during that time.
Just be thankful you are experiencing the emotions you are. Many people grow numb to the emotions and that can cause far more harm than feeling guilty.
I think post 2 on this thread say`s exacly what all the other positive posts say , bad karma will not come your way because you killed a defenceless kitten in extreme pain , you deserve good karma for ending it`s pain . Sleep easier in the knowledge that the distress you suffered from this incident shows you have compassion & a heart .
Stay green .