Spence was killed off-camera immediately after the team left
He just demonstrated that A) he'd lied to a terrorist wing of the IRA and B) he's kind of an idiot. Even if Dierdre was willing to let him leave alive (unlikely), Seamus certainly wouldn't be. Dierdre made a show of bribing/threatening him into silence because, if she'd just executed him right there, the rest of the team might think they'd
turn into a cost-cutting exercise after the heist and the whole plan would fall apart.