Bad Boys II's Ending Finally Explained

When Mike and Marcus search the Palm Mortuary after hours, they are able to know with certainty that Tapia is the one responsible for distributing massive quantities of ecstasy to the city. As a result, Mike, Marcus, and their superior Captain Howard, all devise a raid on Tapia's drug mansion to bring him to justice.

However, as the Miami PD is carrying out the raid, unbeknownst to them the DEA is simultaneously involved in another investigation at the Tapia mansion. The DEA has Syd undercover with Tapia, and she is wearing a wire to capture him saying incriminating things. Though Tapia doesn't discover the wire, he works out Syd is an informant, and the reason why is related to her brother Marcus and his partner Mike.

Two things tip off Tapia and his men to Syd's undercover identity, and both of them are due to Marcus and Mike. First, Tapia's men recognize Marcus and Mike as the fake pest exterminators and realize they are cops. Second, the Miami PD raid occurs while Syd is inside, and it also ends up tipping off his men that something has gone awry. Without trying, Marcus and Mike completely blow Syd's investigation by being too overprotective. It winds up getting Syd kidnapped, where she almost is murdered.