In my view Bhishma was loyal to the throne. He sacrificed everyone and everything on the altar of his sense of duty. He himself did a lot of unethical things in the pursuit of his duty.
He very well knew that his father Shantanu is wasting his life due to his his obsession with another woman but he went ahead and got her married to Shantanu vowing not to get married ever and never claiming the throne.
He served under Chitrangad and Vichitravirya and went to the extent of abducting three girls against their wish so that the Kuru clan can get a heir. Eventually this act was very much responsible for his eventual decline.
He didn't protested vehemently against the disrobing of Draupadi or didnt order to end the game of dice. His pleas was easily ignored.
In the Udyog Parva, Bhishma was one of the rare people to witness the Virata Rupa of lord Krishna but still he didn't made efforts to stop the war. He even participated in the Kaurava attack on Viratnagar during the Agyaatvasa of Pandavas.
So expecting Bhishma to actually support Pandavas was futile. He was a principled man only in theory, in practice he was very much a slave of the throne and he never had the spine to put his foot down