Do you (or God) think that you have to be a christian/theist in order to do good like not kill and love another?
In general, yes, I do. Obviously countless atheists love, and are kind and charitable. But their denial of their Maker, implies both a defiance and oblivion within their character that anticipates a wickedness either latent, or previously exposed.
Does God think that atheists are bad people because they don't believe in him?
Yes, of course! For His universe testifies to both His presence and power - life as we know it, and the earth itself, are miracles. Those who chose to deny His existence are not ignorant of His presence, but prefer to not adhere to His injunctions.
To anwer your (hypothetical) question (I don't have children): I would care that they follow the rules. Not because of my "authority", but because the rules would be good for them. We all know that children will not always follow the rules. That's not because they are bad people an I wouldn't consider them as inherent bad people. I wouldn't fault them for that.
I would. Anyone who does not do what is good for them is perverse and degenerate - they enjoy the evil.
That's an extreme, but the principle is an axiom. Only rebels prefer to challenge authority and the laws of nature - they constantly indulge in what will potentially harm them - excessive hedonism, vices, gluttony, physical abuse, playing with fire, etc....