Celestia has had to deal with her fair share of dissent and ponies wanting to splinter off from the whole of Equestria. Usually, it's a peaceful affair. However, if that group becomes violent and kills other ponies, there is no mercy for them. Celestia's general approach to war is that to end it as quickly as possible with the least amount of loss of life for her own ponies. If that means ruthlessly crushing a violent group, it will be done.
So when a cult of unicorn supremacists with a penchant for dragon symbolism started attacking pegasi and earth ponies and kidnapping unicorns they deemed as good blood, they signed their own death warrant. Then, knowing the Equestrian army was on their tail, they went and made it personal, killing a dear friend and former lover of Celestia. So instead of leading from the rear, commanding the movements and providing the columns of sun fire to weaken the enemy, she left the command to her general and, after the initial barrage of magic and sun fire, took the field herself. She was intent on killing the leader PERSONALLY.
It was one of those moments where she could never know if it would've turned out better or worse if she had swallowed her grief and stuck to her original plan. There was the added benefit of reminding other potential enemies that she is not a pony to be messed with, but it was a double edged sword, also instilling some of that fear into her own ponies. One of the many 'what-ifs' in her life.