"But he’s happy because he’s saving the children!…”
1 - This is Ten saving the earth by destroying the Racnoss in The Runaway Bride.
He’s saving 7 billions of his deeply beloved humans. Lots of children there too.
And yet, he’s not celebrating at all. Actually, he’s borderline suicidal.
Donna has to save him.
Genocide is not fun.
2 - And this is Ten saving Gallifrey by destroying the Daleks.
All giggling, and jumping around with joy.
No guilt, no rage, no remorse, just blatant out of characterness.
And since The Day of the Doctor is supposed to take place after Waters of Mars, Ten’s behaviour is even more OOC.
In Waters of Mars, Ten painfully learns that changing history to choose who to save is wrong, is dangerous, is hýbris.
And suddenly, in The Day of the Doctor, he’s like ”LOL let’s change history, hýbris is cool!”
To me, that’s NOT Ten.