What do you do when you know that the end is inevitable, and you are all set to embrace it? Consider, for example, a person suffering from a painful disease and he/ she is known to die; the only thing unknown could be the time frame he/ she has to bear the suffering. Most of the people would prefer ending the suffering, even if it entails something like death. The earlier the better.
But there is an example in the world history where such suffering was willingly elongated: the night between 9th and 10th Muharram---despite having no water for 3 days and being very clear about his choice and the events which would follow, Imam Hussain (alaye-hissalam) requested the Yazidi forces to allow him and his companions one night's opportunity.
To the apparent eye, the Imam might have wanted to spend his last night praying and offering supplications, which is of course true but there was a side-affect of this one night's delay: one of the commanders of the Yazidi forces, Hur, changed sides the last night, and martyred from Imam's side in the morning---martyred by the forces he once commanded.
Ask yourself: when do you change sides? When the competitor has something higher to offer: a better rank/ authority, more financial rewards, etc. Yet, here is an example of going to the other extreme when Hur made up his mind to get killed in the battle to succeed. The request for spending another night living in Karbala was actually an opportunity to the people on the evil side to make up their mind.
Karbala was a demonstration, and Hur is a symbol of humanity, who got purified by this demonstration.