she didn't make a mistake like show alicent and misinterpret any of viserys's dying words. she wasn't even in the room on the night he died, helaena and her children were, and all he did was tell them bedtime stories and then fall asleep. alicent did what she did for pure self gain

she did not love viserys. she let him rot in his chambers for days while she planned to usurp the throne from his most beloved child and his chosen heir. she wanted aegon on the throne out of nothing more than malice towards her stepdaughter and entitlement that one of her children should be king