When I was younger I most definitely held grudges, and I was also openly vengeful if the issue was large enough. Now, it's not so much a grudge, as it's not active resentment, but I simply don't bother with a person at all if they've crossed a line. Time does not help either.
However, usually a person has to do a lot to reach this point as I give so many chances. If it is a one time incident it depends on severity, intent, and my assessment of their character. Disloyalty, backbiting, blatant disrespect equal automatic dismissal. Harm to a loved one...no way.
I'm pretty sure this doesn't come from the Sun in Pisces though, but the fixed angles, fixed T-square with Pluto in 4th and the Pisces planets' placement in the 8th house.