Can a warrior reach enlightenment? Can someone who fights, even maybe kills or dies, for a cause he considers right,  attain enlightenment?

I wonder that after reading that some Buddhists did not share the Zen samurai's vision at all.
Actually, only warriors can attain enlightenment.  All Buddhas have to die on the cushion if they’re going to wake up.  It takes tremendous courage, fortitude, and self sacrifice in order to make that happen.  Only a warrior possesses those qualities. 

But to answer you question, the one really doesn’t have anything to do with the other.  It really astounds me how often people forget that the same temple that gave us Zen is the very same one that brought us the 8th century equivalent of the Apache helicopter.  Shaolin monks were no slouch with the sword.  Many of them fought, many of them died, and many of them killed.  Many of them were also deeply awake.

I hope it’s fairly obvious what distinguishes the Shaolin monk from your average mass shooter, John Rambo wannabe, or even the samurai. To put it plainly, the true Zen warrior wholeheartedly follows the dharma.  All of his decisions are made facing emptiness.  Mu doesn’t tell you blow up buildings, pick off people in the street, or to be a solider of fortune.  Shunyata isn’t a lord one serves or a stilted code of beliefs one takes up to defend.  Seeing emptiness, the zen warrior feels compassion.  His sword is drawn with compassion.  If he acts with violence, it is to further that compassion even if doing so destroys him inside.  The real warrior of zen serves mushin and nothing else.