So far, the zombie outbreak hasn't fundamentally changed Rick's character, but "TS-19" makes it abundantly clear that post-zombie Shane is antithetical to what he used to be, and getting worse. The Walking Dead can be a little on-the-nose at times (did we really need the shot of Shane swigging from a whiskey bottle in the shower?), but his drinking leads to a believable, ugly scene in which Shane aggressively attempts to rekindle his romantic past with Lori. When she tells him to stop, he doesn't, and it's not until she scratches him that he bitterly backs off. Zombies or no zombies, there's no safe haven for Lori as long as Shane's around.
Meanwhile, Rick, who's similarly drunk but far less violent, has a lucid conversation with Jenner. Rick has been unwaveringly hopeful and courageous around others this season, and it's telling that Jenner is the person to whom he finally voices his doubts. Though it's not immediately apparent, Rick and Jenner have a lot in common: complete devotion to their loved ones, and a soft spot for strangers in need (think of Rick's sympathy for the Vatos in episode 4, and how closely it echoes Jenner's sympathy for the survivors in the first half of this episode). It's even apparent in the way they dress. Like Rick, Jenner wears his uniform (in his case a tie and lab coat) despite the complete breakdown of conventional social structures. There's an important, symbolic message in keeping traditions alive, because those traditions affirm what makes us human. Rick and Jenner both understand that.
Jenner refers to the zombie outbreak as an "extinction event," and he locks the survivors in the CDC, which is conveniently set to self-destruct the day after they arrive. But Jenner isn't a villain—he genuinely believes that he's saving the survivors the pain of a struggle they'll inevitably lose. When Jenner prophetically tells Rick that the day will come when he won't be grateful for his "chance" to live, he's speaking with the experience of someone who's lost everything. If Rick loses Lori or Carl—as Jenner lost his wife (the zombie "test subject 19" who gives the episode its title)—Jenner's comments will carry a weight that Rick can't possibly understand right now (I'll leave you to speculate about Jenner's last, mysterious whisper to Rick, though I politely request that those who have read the Walking Dead comics refrain from spoiling future plot developments).
Rick successfully convinces Jenner to let them go, and the group races to the lobby. Still reeling from the loss of her sister Amy, Andrea wants to die, but Dale won't leave without her, so she follows him to escape (poor Jacqui, who also elects to stay behind, doesn't have a kindly old man to talk her out of suicide by immolation). The CDC's front doors are locked, but—in a variation on Chekov's proverbial gun—the grenade introduced in the second episode goes off in the sixth (intriguingly, Carol has been quietly holding onto it since she found it in the laundry, which would seem to indicate a more calculating, self-centered side to her character than we've seen). The CDC explodes, and the survivors (minus Jacqui and Jenner) get back in their cars to seek safety elsewhere.