[spoiler]It really was to be expected honestly, and it's not the honesty of her feelings that is the problem with her, but the way she expresses them. She really goes TOO far, back when he chained Yukki to that chain so that they'd stay together forever and now when she tricked him when he was weak mentally to get rid of Hinata.[/spoiler]
[spoiler]See, I'm kinda working under the train of thought that she started out as a normal girl with slight stalker like tendencies (this was mainly due to that one chapter where she ran around the place intercepting Yuki at the amusement park; she showed that there was that side to herself normally, but there wasn't any real time where it struck me as being WTF creepy like the times during the main storyline). Things got amplified by 100 after she got involved with the Future Diaries. My current theory is that originally, Yuno met Yuki according to her flashback and things preceded to go along as per her flashbacks (she got abused and caged, but continued to grow closer to Yuki in the time she had to herself), until she got recruited to play a part in the game with him. All of this however, took place in Yuno's
first playthrough in the survival game. The manga's storyline is actually
her second time through.
(There's a bunch more I'm thinking of now, but I want to post cliff notes right now since I'm kinda out of it.
)
Now then, what I think originally happened is that Yuno actually was contacted by Deus & played through the game to its climax and became "god', but in doing so, she wound up killing the Yuki that existed in her first play through, triggering a nervous breakdown on her part. Upon doing so, she believed that she could save the folks she killed, much like how Yuki in the main series believes he can do the same, and when it was revealed that things like that can't be, she broke further and took the second option, that reversal of time flow to when all the people existed. When she came to in this time-looped world (let's call it Play through 2), she initially found herself still the same, but complications came up in that she existed as a individual who already existed in that time frame, the same Yuno that had just recently come to love Yuki. Sometime after this came up, she was then contacted by Murumuru outside of Deus' knowledge, to play once again. Murumuru then had the Yuno of that time line and her family murdered (by Yuno from Play through 1) so that there wouldn't be any issues in the cause & effect horizon and things started from there.
Yuno's crazy devotion to Yuki and general derangement is a byproduct of her own past traumas as well as her experience with the first survival game. Yuno knows exactly what happens to one who plays the game (hence her instability and inability to trust other people), and knows from first hand experience that this is the inevitable outcome. Because of this, she is monstrously overprotective of Yuki, hence the ax craziness, her ability to kill without a second thought, and that one time where she kidnapped Yuki and kept him away from the outside world -- she was trying to protect him from the diary holders, world, and the game itself in an extremely WTF obsessive kind of way. (Even this is a stretch to me, because [tornado fang] me that was disturbing, but when I think of it, this might hold some weight.)
This theory being true would also explain why Akise's naturally prying nature unnerves her so much that she has made it a point to go out of her way and kill him -- if this all were to get out, it would probably change the nature of whatever salvageable future with Yuki she has and that in her mind is the #1 thing she doesn't want happening. It also explains why she was able to go and murder the 11th, she
is Gasai Yuno, just not the Gasai Yuno that should have been there for the second play through in the current universe. Finally, it might actually shine some light on why she is able to sign her life away to Yuki, there might actually be some part of her so distraught by what she did to Yuki in the first playthrough, that she is determined to let Yuki win and "use" her to his advantage as some sort of tragic penance.
The biggest holes that need filling in this pet theory of mine are what exactly Murumuru did with Yuno & the Gasais and what sort of intents she has in doing this all.[/spoiler]
TL;DR: Yuno isn't malicious naturally, she's this way because the game [tornado fang]'d her up.