I have to disagree with you on primitive people not using a knife as a weapon. Actually it was and in many places still is the primary weapon and also the badge of a man. Even though the natives here all have access to guns they stab each other far more often than they shoot. Probably 10 to 1. My towns Penitentary
had a huge prison riot yesterday between 3 native gangs which are split down tribal lines.

Philippines people have a very long tradition of knife fighting that is still practised and quite popular among hard core MBC guys. The traditional syle is considered to brutal even for some army type units because they spend a lot of time teaching how to kill people who are unarmed or not expecting it and even how to best kill a sleeping person. This is perfectly acceptable because any man without a knife is considered an idiot and it is their own fault.

I studied some of the Philopino MBC but I don't like the reverse grip or the low line so a lot of it was not to useful.

Probably the best style of knife fighting to learn is the combatives developed for the WW2 commandos. Col Applegates book "Kill or get Killed" has some excellent advice and is free and legal to download because it is a US military manual. You can find it at "steve's pages".

They use the FS Dagger which is an excellent weapon but I think there are some better choices now. One of the best inexpensive fighting knives is the Buck 119.
Most fighting knives are either good stabbers or good slashers it is very very rare to find a kife that can do both. The 119 is one of those rare knives although I would wrap a bit of hockey tape on the handle.

My current fave concealed fighter is my Mule. A WW2 commando who had confirmed kills of several different men said that you did not hack like a baseball bat or stab but cut preferably the carotid artery beside the wind pipe.

A good knowledge of anatomy is crucial. In a current big trial in our town their was a prison murder(our prison is pretty hairy)committed on behalf of the Hells Angels the victim a former flaming gay contract killer(you can't make this stuff up) was stabbed 184 times by 2 men one of whom is claiming self defence.
If you aim for specific organs it should take a bit less work. To be fair they had very small prison shanks.

A good knowledge of anatomy can also save your life. If you are on the ground and you are being kicked there is an artery on the inside of each leg to aim for and it is like cutting the bottom out of a styrofoam coffee cup.

You don't need or even want a thick knife for a fighter. Most Scrappers are not very good fighting knives. The DFLE and the SS4 are the closest. One knife I just saw that would make a wicked fighter is the Microtech filet knife. It is a huge thin razor blade of a knife. I may pick one up.