Alfalfa does kidney damage to older horses. I have seen, first hand, an older horse slowly die from his kidneys shutting down due to an excess of alfalfa hay and oats. He first foundered, and from there his quality of life deteriorated extremely quickly. Because of the "over-work" the kidneys were doing, there was a constant stream of urine from his stall to the drain in the middle of the barn no matter how high the shavings were piled in his stall. He was drinking nearly twice the normal amount of water than a usual horse and it was not even hot outside (about -10C). He could not stay hydrated, when he chewed his eyes moved in and out of his skull (a sign of severe dehydration).
The vet was called after weeks of the horse being in agony after he foundered (and still was being fed a diet of alfalfa and oats) and the decision was made to put him down. The horse could not walk, and for the last few days he would not eat or stand up and was not urinating at all. He lay in his stall and groaned in agony. The horse was not even 25 when he was put down.
Most older horses have some type of Insulin Resistance or Cushings. Because of this, no horse older than about 20 (in my opinion) should be fed any alfalfa. They should strictly be on a low protein, high fat diet. A horse who is actually bad enough to be diagnosed with either of these conditions, could potentially die from only a few meals of alfalfa hay.
Princess warrior, although it is not for sure that alfalfa was the main cause in your horse's death, it was probably at least a small factor. Next time you need to put weight on an older horse, please consult an equine nutritionist and have your vet run tests before putting the horse on a high protein diet, like alfalfa.
As far as making horses hot, it really depends on the horse. Just like people, they will respond differently. It is more the sugars in the hay that are going to send some horses off the handle hyper, just like some people, if you give them sugar they go bonkers. Other horses and people can handle the sugar just fine. It is an insulin thing.
My horse is fed a 30% alfalfa mix and also gets oats because he is in such had work he needs the energy provided by the alfalfa and oats. He responds fine to this and does not get hyper, it just gives him more clean burning fuel :P For a pleasure horse, I would not feed excessive alfalfa or any oats. Some horses who respond badly to alfalfa can't have any. Just like any other thing with horses, their program needs to be individually tailored to them. Never treat horses as "this works for everyone" because it usually never works that way, and when in doubt, call a professional. Spending a few hundred dollars to prevent a few thousand dollar vet hospital stay, or having to put your horse down, is always worth it!