It's normal to be in a relationship and not see problematic issues clearly. That is especially true in the beginning of the relationship when you are falling in love. Sometimes it takes friends and family to point out when a relationship has become less than healthy. Remove the blinders and pay attention to the warning signs of a bad boyfriend. A relationship that has physical, emotional, sexual abuse, as well as controlling behaviour, are warning signs to leave the relationship right away.
If your boyfriend has raised a hand like he's going to hit you, or has intimidated you in any way, that is a warning sign of physical abuse to come. If he breaks your things, shoves you or slaps you, the is the warning sign of a bad boyfriend and you must leave, as you're in danger. It doesn't have to be actual hitting. Just the thread of physical harm is abusive.
Verbal abuse can start small and escalate quickly. If your boyfriend criticises how you dress, your weight or your looks, that is a warning sign. It often escalates further as he says things like "you will never be able to find someone else" due to looks or weight. That crosses the line into verbal abuse.
A bad boyfriend might be pushy when it comes to sex. If he wants to be intimate with you after just meeting you, and pressures you intensely, that is a warning sign that he will be a bad boyfriend. If you are in a relationship and he says that you don't love him if you won't have sex with him, or gets angry, drop the boyfriend.
Controlling behaviour is something you should not tolerate. It often begins with him checking your phone call or e-mails, cutting you off from friends and family. It can even escalate to spying on you and stalking you. Someone who loves and respects you will not treat you that way.