What motivation did the popes offer to people going on crusade?

He promised to absolve all crusaders of their past sins and that all crusaders who died in battle would go to God's side.

Sorry, no pope has ever promised to absolve anyone of their past sins ahead of time, nor said that if they died in battle they would go to God's side - that sounds like something the Muslims would dream up. The popes did offer a plenary indulgence for anyone that went on a crusade. But a plenary indulgence (which are still offered today) is only good for someone who has *previously* been to confession, been absolved from their sins (NOTE: this is a PRIOR CONDITION), then they must pray for the intentions of the Holy Father, and go to Holy Communion AND they must be FREE OF ALL ATTACHMENT TO SIN. The Holy Father has always been able to offer a plenary indulgence, and there are numerous plenary indulgences available today. And most people can fulfill the three primary conditions of confession, prayer, and Holy Communion. Few people can fulfill the last one of freedom from all attachment to sin.