In the episode "Ghosts", Dante tells Siddiq that he was a field medic during combat in Iraq and suffered from PTSD after losing his entire military squad.
After seeing Dante burying one of the older infected women that he had grown close to, Siddiq attempts to commit suicide by drowning himself, but is rescued by Rosita.
As they talk, Siddiq has a realization and rushes to test the water supply, discovering it to be contaminated; he blames himself for failing to notice.
After decontaminating the water supply, Dante tries to cheer up a depressed Siddiq, reassuring him that what happened wasn't his fault.
In the first part finale "The World Before", it is shown that in flashback several months after the blizzard, Alpha chooses and assigns Dante, due to his loyalty and because Lydia doesn't know him, to infiltrate Alexandria as her spy and sabotage the community from the inside for the impending war between them.
It is revealed that Dante is the one who painted the "Silence the Whispers" graffiti, sabotaged the water supply, and suffocated Cheryl to death.
Juan Javier Cardenas was cast as Dante, Siddiq's wise-cracking medical assistant and a spy for the Whisperers.
[1] Angela Kang explains the decision to use Dante as a spy: "We just talked about the fact that a lot of times sociopaths can be very high functioning individuals that hide in plain sight because they're accomplished.