According to the comments on the video, he smashed it to make the pulp softer and easier to eat.
Also rubs the juice on his body to ward off some insects.
"Oh whats this?"
"I wonder how it tastes like"
"OH GOD IT TASTES SO SWEET AND SOUR!" " **** THIS YELLOW OF FRUIT OF DEATH I PUTTED IN MY MOUTH!" "wait come back!!!...."
1: Monkeys rub citrus fruits on themselves to repel certain bugs.
2: Hes bashing it to soften it, if you watch the full video, he keeps bashing it, biting a chunk, then bashing, then eating, repeat.
Sorry to ruin your Overdose of D'aw, Monkeys aren't adorably ignorant like puppies/kittens.