I agree 100% that euthanasia should be legal. With the latest proposals going around though I think someone with dementia would not be allowed it (must be of sound mind, etc).
I saw my old man endure 3 months of utter hell with throat cancer before dying. He had other illnesses before (he was in severe heart failure for about a year) but cancer came along and just took his entire quality of life. His neighbour had died the year before of a sudden heart attack, and I remember my dad saying that the neighbour had "won the golden ticket". A far better way to go, he was thinking, than enduring what he was going through.