Lyxen
Great Old One
killing Orcs is an evil act now?
See the other answers, it might depend a bit on the circumstances but killing foragers (assuming that they are only that, and not the brutal orc warriors who have just destroyed your home town two days ago after torturing and killing all your family and friends and who are now just foraging because they need it and have not other town to burn around the corner) in cold blood and torturing certainly is evil.
Id argue unprovoked mass murder of a bunch of people foraging in the woods, and brutal torture or a survivor, in order to locate the rest of those people, in order to engage in some light genocide, is a bit more than simply 'inconsistent with the CG alignment' but I otherwise agree with your post.
It depends on the circumstances. The guy might absolutely hate orcs for background or history reasons. These "foragers" might be doing just that now because they don't have a town to slaughter, pillage and burn. There might be too many to deal with otherwise due to the points above, etc.
Circumstances always matter a lot, and this is where most of the alignment discussions fail, not because of general principles, but just because the event and the circumstances are not described precisely enough, so people have different views of the even.
Personally, I'd let the player know then and there of this, the instant he told me he was going to massacre them, that's the fact what he's about to do is evil, that this fact is absolute and the view of the Gods, and to pull their head in because (as a non evil, heroic campaign) we're not doing that kind of stuff at this table.
Up to you, but see above about circumstances.
Do it, and your PC is obviously not Heroic, nor Good, and (according to the restrictions of the game) not a PC anymore.
And then again, it depends on the circumstances.