They almost always run away when they believe it is suicide to stand and fight and wish to live to fight another day, unless...
They are enraged with bloodlust (then they may not even know they are losing), they are protecting there home oe women and children (if they care about such), running away from battle is against some deep sense of honor, against their religion by Crom, death is better than your BBEG boss making an example of you as a deserter, your body is likely to be recovered and you'll be raised, etc. Those are mostly "bad guy" motivations for standing their ground to the death.
Sometimes they surrender instead if they feel they may be given quarter or they are too valauble alive. On the latter they will cetainly explain this while defednding themselves and I've had one evil captain level leader intentionally wasted his last few ogres against the PCs why? Well it was likely the PCs were after information on his masters plans, with all the other flunkies dead he was the only prisoner they could take back for interrogation. He was forthright with some local info to save his skin, but held back on more strategic info to retain his value and life.
I give full exp for foes that run away, and you get exp again if you face them again and get them to run away. You also get full exp for taking prisoners, any thing that defeats or overcomes the challenge.