And as for Willow, she only went haywire after the Glory deal. Her relationship with Tara was slowly spiralling out of control due to her magic addiction. She tried to in turn
use magicks to heal this problem... Tara found out... she left Willow and Willow went into a depression state. It wasn't until she turned Amy back into a human (formerly a rat, since season 2. Willow never knew how to do it) during this state that she began to become seriously addicted... I mean drug-type deal. Upon Amy's return, Amy wanted to go out and have a blast (she's a witch also) with her powers. Willow got dragged along. It started out with simple things... transfiguration in public areas... conjuring, etc. Until Amy told Willow about a powerful Warlock dwelling in Sunnydale who fuels witches with power, which acts like a drug high and gives you a complete amount of magicks and energy. Willow soon got addicted to it. It was during this phase that Willow took Dawn out to 'go to a movie' and stopped off at Wrack's dwelling (the warlock) and got a fuelling of magicks and lost control... which resulted in Willow and Dawn having a car accident and being assaulted by a demon. Buffy knew something had to be done, so did Willow. Willow went cold turkey...
After Willow and Tara got back together... and everything was going great with them, Warren (little psycho nerd... Buffy's 'arch-nemesis...is) came rocking up to Buffy's place, and shot Buffy in the chest, then ran off still firing random shots behind him. One of them shot through the window upstairs and killed Tara. Willow invoked all the power she could at that moment and summoned the god Osiris to request he bring her back (Willow had to do a complete ritual in season 5 to do this... this shows how much more powerful she has become). Osiris denied her request and Willow got mad... she screamed at him and channeled a form of magickal energy and banished Osiris (he even screamed "Ahh!"). She asked Xander who had done all of this to Buffy (in the ambulance) and especially Tara. Not like she wasn't powerful enough at the time... she went to the Magic Box and blew the door open (the lights smashed as she walked past them) and absorbed the knowledge and power from every single black arts book the shop held, which turned her hair black and sent her on a murdering spree (to kill Warren and his two comrads).
That's my input.