Was I a bad cat owner?

I used to live in Phoenix, AZ. My neighbors had a cat who lived outside and had lots a litter of 5 kittens. The mama cat had her kittens on the streets and they grew up there. Because of that, 2 got hit by cars. One must've gotten in a fight or attacked by adog because he had a infected and broken looking leg. He soon died. One day, we saw the neighbors who owned the cats, moved out. They took the mom and left behind the two remaining cats. My mom took the cats in. (boy and girl, such sweethearts! They were 2 months old and adorable! :)) anyway, a year later we moved to Pennsylvania. We moved to the country side where there was nature all around. Since the cats were born outside cats, they always ran outside a lot. So much, after a while, whenever they would sit at the door, we'd open the door and they'd come back an hour or so later. Don't judge, they didn't come back hurt and they stayed close to home, NEVER ran away. They seemed happy outside, and even in winter they'd some times go out. But when my little boy(Tony) was two, he went out and never came back :'( he got hit by a car :''( I still cry, but (please be nice) was I being a bad owner? They seemed happy outside, they had lots of energy and they'd be bored stuck in the house all day. We took care of them and I loved Tony like a son, his sister (Holly) was my sister's cat. I know he was only 2, but he was born an outside cat. His sister holly is still alive and very healthy, she will be 3 on July 4th. Sorry, this is long. So was I a bad owner? And what can I do to make sure holly stays with my family for a while? Danke and gracias.