Well Crawdaunt, another Water/Dark with similarly aggressive, combative no-holds-barred behavior gets Nasty Plot. It even has the same high-attack-but-decent-SpA situation as Sharpedo, so it does seem odd that the shark doesn't get the move...
Crawdaunt shows some sense of intelligence. It's not super aggressive because it's competing for food or a mate, the dex entries just make it out to be a bully who greedily wants an entire lake/pond for itself. Also, when ITS in a weakened state (has just molted or one of its pincers break off), it turns cowardly and hides away until it's strong again. It's fighting because it like to fight, it's showing intelligent intent to cause harm. Crawdaunt be like (speak in your best mobster voice): "This is a nice lake/pond you got here, I'LL TAKE IT! EVERYONE OUT! Huh, you wanna fight me? YOU wanna fight ME. Do you know WHO I am? PINCER TO THE FACE! GET OUT AND STAY OUT! Yeah, who's next? I see you hidin' over there! OUT YOU GO! EVERYONE gets an eviction notice! Oh, ANOTHER wise guy, huh? PINCER TO YOUR- OH SHI! My pincer broke! Ugh, ugh, YOU GOT OFF EASY! Gotta hide, gotta hide...".
Sharpedo is just an eating machine. It sees anything it considers food, it first charges at them at full speed possibly trying to swallow in one bite or give a big crunch before the opponent knows what happened. Then, if it doesn't eat its prey in one gulf, it viciously flails and bites until either it's knocked out/forced away or kills its prey and eats it. Then it goes about it's business until spotting the next thing it wants to eat. There's really no thinking, no plotting, just "EAT. EAT. EAT. EAT. EAT. EAT.".