When it comes to aces in the hole, not much is better than dragons. They ensured the victory of Daenerys's ancestor Aegon the Conqueror when he invaded Westeros, and they ensure a victory for Dany pretty much every time — you mess with Khaleesi, you get burned. But then the Night King ruins everything (as usual) in season seven and kills Viserion right in front of his mother and siblings. As if that wasn't bad enough, Viserion is turned into a zombie ice dragon and freezer burns a huge hole in the wall. Not cool.
Now that Dany has vowed to help Jon destroy the Night King it's only a matter of time before she faces him again, which means it's only a matter of time before she finds out one of her babies is now an ice-breathing creature of evil. We've seen the damage zombie Viserion can do, so the best tactical move would be to take him out ASAP. Will Dany be forced to make Rhaegal and Drogon kill their brother? Will she have to watch Viserion die a second time?