Venison fat will leave that nasty taste. It will turn rancid. I trim allow it off my deer meat before grinding. I add beef trim to my pile for burgers. Sausage gets pork added.
We like 15% fat. Prefer pork but beef will do. Our local supermarket gives it to us for no cost. 15% is enough fat to hold the burgers together but not enough that it runs out of the burger creating a mess.
Your tastes may vary.
If you (we do) are using pork butts when making sausage or bacon they are roughly 30% fat.