Why was doc brown so hesitant in telling marty he was going to fukk up his own futute?

[cole phelps]

Marty came all the way to the 1850s to tell doc he was going to get lit up by biffs crazy ass ancestor and showed him proof but he couldn't do the same and tell marty he was going to do something stupid and fukk up his hand and his life in the process man if i was marty i'd be hot if this was kept from me. was doc trying to teach him some kind of lesson or something?