[Kyureki]

For a few examples of the "ruthless" strategies the MC employs which lead to him being called a demon/devil...

Spoiler:

In a defensive siege-like battle, he had a giant empty moat around a built-in-a-few-hours fort (used that world's magic to build it so quickly, which in a world where magic is used solely for combat, baffled the enemy out of their wits and there was a side note about this bringing about new innovation) beside a giant river, and once the enemy had filled up the moat to a certain point, he had the breakwater on the upstream part of the moat taken down to flash flood the moat, and since very few people in this world know how to swim, thus drowned a very large portion of the enemy army to death and any survivors/prisoners were all killed.

That started the MC's fame as a demon/devil in the surroundings (which he purposely spread rumours about to reduce the number of civilians that would be drafted to fight). The MC then makes use of his infamy later for the final battle where he spreads rumours that his faction sent people to burn the opposing nobles' territories to the ground. This was of course fake since that would weaken the country too much post-war, but due to his infamy for being so ruthless, all the nobles that had gathered together for the final battle ran back to their territories to protect them.