>>534989338
The cow wasn't actually there, they were inside the fathoms remember? Dongrang just inherently assumed that if they were to meet again, the cow would not recognize him, because deep down he sees himself as too far gone and not remotely close to who he was as a person in the past.

So instead of merely enduring the grief and disappointment of never getting anything from such a fact, he decided to move on and just become wholly inhumane and uncaring towards anything he has to do to achieve his dream of making his own great invention and getting recognition for it, so he kills the cow, signifying his new drive to kill his past and anything that might remind him of it so he can move forward.