Bhishma was given the boon to die whenever he wished to. After seeing all the atrocities against the Pandavas, why did he not take his own life and spared them the trouble of fighting against him?
Bhishma Pitamaha was very well versed in all Scriptures. So, he knew that suicide is heinous act. He himself has mentioned this at many places.
Rudeness of speech, fierceness, severity of chastisement, infliction of pain on the body, suicide, and frustrating one's own objects, these are the six kinds of faults born of wrath, that have also been mentioned. Mahabharata, Santi Parva, SECTION LIX
A deceitful person, falling away from all duties and abandoning those of his own order, always wishes to betake himself to the practices of Asuras for supporting life. Such a sinful wretch living by deceit should be slain by every means. Such sinful men think that there is nothing in this world higher than wealth. Such men should never be tolerated. No one should eat with them. They should be regarded to have fallen down in consequence of their sins. Indeed, fallen away from the condition of humanity and shut out from the grace of the gods, they are even like evil genii. Without sacrifices and without penances as they are, forbear from their companionship. If their wealth be lost, they commit even suicide which is exceedingly pitiable. Mahabharata, Santi Parva, SECTION CIX
So, he choose to die in battlefield because for Kshatriya, it is highly applauded. His views on death in battlefield is as follows:
"Indra said, 'Regarding this Sudeva, O sire, the great sacrifice of battle had often been spread out by him. The same becomes the case with every other man that engages in fight. Every warrior accoutred in armour, by advancing against foes in battle array, becomes installed in that sacrifice. Indeed, it is a settled conclusion that such a person, by acting in this way, comes to be regarded as the performer of the sacrifice of battle.' Mahabharata, Santi Parva, SECTION XCVIII