Like the Joker once said "madness is like gravity, all it needs is a little push"
Family has been inbreeding for centuies, House Motto is "Fire and Blood"
Has repeatedly said and threatened to burn cities to the ground. Has never reacted well when feeling disrespected, slighted or shown the proper respect. Has repeatedly shown you have only 2 choices follow her or die, it's her way or the fried way. She's had advisors she trusted that helped curb some of those dark impulses, Daario, Jorah etc. Right now, she doesn't have people to advise her that she trusts. Even leading up to the battle she said she would save future generations at the expense of the current one. There were plenty of other signs, but how many more do you need?
Like Olenna once told her ".....you're a dragon, be a dragon."
Dany was always a Khaleesi and they are savages
She ripped thru men without regret and she always gave into her rage with violence
Let Drogo kill her brother......conveniently she is now heir to the throne
Burned the Witch
Burned down House of Undying and locked up Ducksauce
Killed Masters in Yunkai
Crucified 150 masters in Slavers Bay
Killed assassin in Mereen
Killed the enemies at the Parlay when Mereen revolted
Killed all the Khals
Killed the Tarleys without mercy
Made Jon Snow bend the knee and didn't return the gesture after learning the truth
She was truly alone.... she lost Missandei, betrayed by Jon, Varys and Tyrion
She saved the North and no one even thanked her
She gave into the Dark Side and became consumed by it and it made her indestructible.....sorry wrong series
Repeatedly threatened and banished Tyrion, Jorah and Varys