Yes, babies who die after baptism go straight to heaven. They have been freed from original sin and are not yet capable of personal sin, so there is no impediment for them to being admitted to the immediate presence of God.
No, it would not be better for babies “to be killed so there’s no chance they go to hell.” God grants us free will, and with it the possibility of refusing him, because he wants us to be capable to love him back and to follow his commandments for love of him. To take away from any person the ability to choose to love and serve God as a conscious, freely-offered gift of self would be a grave evil.
