“I guess a spontaneous reaction is all of a sudden מותר”
Who said it is “mutar”. The point was that except for highly disciplined individuals such as David Hamelech and Coffee Addict, a spontaneous reaction is one that typically doesn’t involve prior thought and halachic analysis. On hearing news of the destruction of someone who has caused incredible death and destruction, its a normal human reaction to show joy and even c’v a few dance steps before pausing to consider the appropriate response protocols.