In the name of religion.

Because they accidentally drove into your rural driveway

A few years back it snowed in Seattle, which is uncommon. I saw a video where folks were having a snowball fight, as you do. A snowball entered the partially open window of a driver. The lady proceeded to Hop the curb, and drive up some embankments trying to hit people with her car.

For a fuckin snowball.

Because someone in your government has decided that you have to.

According to Johnny Cash it's "just to see him die".

Pol Pot mass murdered anyone wearing glasses.

Revenge. How should killing a person make it better?

And war, some rich people profit, normal people die

because they're mad at the person for being gay, trans, or a different race or religion than you.

Because they don't have same color skin.

Well if you kill everyone, will there be anyone left to shame you? Therefore, it wouldn't be dumb to kill people then.

Mistaken identity. "Sorry, my bad."

We get in a fight at a tripple crossroads. He dies. Then I go back to his kingdom and marry his wife. Little did I know she was my mother and he was my father...

To maximize shareholders' profits.

Racism

Hold on.

There's reasons to kill someone‽

You want them to live

Because you were not paying attention while behind the wheel.

For ringing your doorbell

A frozen yogurt (no joke, almost happened at my high school)

Well, the dumbest reason I've seen people get murder-y for is typically fighting over inheritance.

It's like... now there's even more inheritance to fight over. Then also you just paid for one funeral, and now you want to pay for another?

In self defense after someone summoned Bigfoot to kill you.

Because in your impaired judgement you were 'good to drive home' after a few drinks.

Because you want to know what it's like to kill someone.

because they were unable to speak

Because you had to eat without a table.

Looked at me funny

Politics.

Followers stake the moral upper ground and turn otherwise awful douchebags into martyrs.

For whistling.

There will never be a dumbest reason to kill somebody. Humankind will always be inventing the next dumbest reason.

I feel like you were going for silly answers and got depressingly true answers. :(