The poor beast suffered years of physical abuse and neglect, can you blame him for reacting poorly when he finally got his shot at freedom? We don't blame the abused dog if it bites the mailman, can we blame DK for grabbing Mario's girlfriend, Pauline (safe to assume also an animal abuser, if only by proxy) and simply running away? He didn't even hurt anybody, he just ran. And we all know what happens next: Mario, possessed by the invincibility of rage, hurdles every obstacle in the pursuit of his frightened pet -- barrels will not stop him, fireballs will not stop him, ramshackle construction sites will not stop him, nothing will stop him, not even death (he's got extra lives) -- until he slowly but surely chases down the cornered, abused, terrified monkey, and drops him from the top of a skyscraper.
Teaching children around the world a valuable lesson about pet care.
We should probably tell you that the Donkey Kong of today -- the one seen happy and healthy in all the current Nintendo games? Rare made a very specific point of mentioning that he's not the original DK; that's his son. Now, they're not outright saying what happened to the original Donkey Kong, but it's best not to look too closely between the lines.
Because you'll probably find Mario there, covered in ape-blood, screaming in unearthly fury.
"When they talk to you, you just fell down the stairs, right?"