> Isn't some C4 good enough?

Not by itself. An area weapon is fine, which is why I said grenade. They are effective even with enemy in defilade. Rifles are cumbersome, give their position away and have recoil.

> why do you need the drone to be autonomous

Electronic Countermeasures

Also, you can leave scores of drones lying in wait, and alert a small number of pilots to fly the right ones when motion is detected.

This is why autonomy might be desirable:

https://duckduckgo.com/?q=turkish+soldiers+drone+jammer&t=ff...