We should be careful to draw a causal relationship here. It might not be that getting the military equipment causes police to be more aggressive and therefore have more killings. It might instead be that the more aggressive police departments are the ones that more seek to purchase military equipment, and the more aggressive police are the ones with more killings. I tend to think that's the case. Regardless, there is…
I agree with the sentiment, but my impression (based on little evidence) is that the "need" here is to create a wider market for these weapons.
It takes money to develop new weapons, and a larger customer base helps defray those costs.