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I think most DMs would probably allow a character to eat the bodies of fallen adversaries, BUT I also expect a revulsion reaction by many gamers...perhaps that was what you were seeking? (Many gamers will avoid playing with you once that behavior is known.) I would also expect that NPCs (played by the DM) should usually see the act as confirming that the gnoll is still a savage beast, not civilized, and definitely not a creature they want in their community. Such actions may trigger consequences which are not as much fun for the group. Remember...this is a group/team activity and the objective is for everyone to have fun.

As others have stated, starting fires in cities for the purpose of burning the city down (otherwise known as arson), will be evil and illegal. I would expect the NPCs to turn very hostile towards that character, and possibly the other PCs by assumption they condone it or support it.

As Shawn stated, there will never be a skill challenge to burn down a city in LFR. This is a shared world campaign and an official setting. We share it with WotC writers & designers. We would have to get approval to put an entire city at risk, as that would remove a dot from their map, make the campaign guide have obsolete information. For some of us (Waterdeep, and Baldurs' Gate), it is our entire region. Not going to happen. Also remember, in a shared world campaign, the results are driven by the majority of tables...so only if more than half the tables did that action would it be part of the official history. That seems very unlikely, even for a small village.

I could see a possible plot where some community plans to clear a large swath of forest, for expansion or a new community, and that might be opposed by the faithful of Silvanus. I think they would act before a city/village was built, before too many trees were cut down, to express their discontent. That would be an interesting combat/skill challenge, in which the characters might be of divided viewpoints if done well.

Keith