Win a two player versus fight in the last Boss' room. Answer "No" on the second question, then defeat the last Boss to see the bad ending during credits.
Bonus lives
Wait until the count is done at the end of each level before proceeding. You will sometimes get more lives.
Extra points
Defeat the level 3 Boss and use the special weapon before he lands on the ground to receive a 5,000 point bonus.
Fighting Abadede
Punch Abadede, the Round 3 Boss, before he hits you when he dashes towards you. He will dash again, so punch him again. Repeat this until he falls down and look out for any other opponents. Note: Abadede makes an appearance in Streets Of Rage 2 as the Stage 4 main Boss in the underground arena.
Fighting Big Ben
Make sure you are out of Big Ben's way when he breathes fire (e.g. if he is near the top of the screen, make sure you are at the bottom so he will miss.) Then, get behind and grab him. If you throw him now, he will collapse on you, so quickly release him. Then, grab him immediately and he should be facing you. All you have to do now is continually hit him until he is defeated. Note: Big Ben makes an appearance in Streets Of Rage 2 on Stage 4 near the baseball diamond. He is a bit shorter and wears a cap.
Fighting the final boss
Play a two player game. When you reach the final Boss, have one player fight the final Boss while the other player fights the other enemies.
Two player versus mode
Begin game play in two player mode and reach the last Boss. The Boss will ask if you want to join him. Have player one answer "No" and player two answer "Yes".
Two player versus without bad ending
Win a two player versus fight in the last Boss' room. Answer "Yes" on the second question. You will fall down a trap to Round 6. Go through the levels until you reach the last Boss again. This time he will not ask any more questions, and will start fighting. Defeat him and the good ending will appear during the credits.
Bad ending...