Members of the Koopa Troop have displayed different powers from game to game. For example, in SMB, the Goombas could do no more but walk around and hope to walk into Mario and kill him. But in SMB3, the Goombas were capable of flight. Even moreso, in SMRPG Goombas were capable spit attacks.
Even King Bowser is not exempt from changes in ability. Initially he was only capable of throwing fireballs and/or hammers. In SMRPG he had all kinds of magical abilities, and in SM64 he could even teleport and create powerful electric currents. Besides all of this, Bowser seems to be able to suddenly change in size.
What's to account for these incredible changes in ability? How can the troops kill with just a mere touch? These are the questions I intend to answer.
The first question: What's to account for sudden changes in ability? As we all know, Bowser collects power stars. These stars do more than look nice. They provide -as their name implies- power. When Bowser collects power stars, he keeps some for himself, bestowing certain powers, and he distributes the rest to his troops. That is why he and his troops seem to gain new abilities with each game. The more power stars, the greater power attained.
The second question: How do Koopa Troopers kill with a simple touch? For the answer to this we must refer to the power stars yet again. One of the abilities bestowed by the power stars is an injuring touch. However, for the most part, if a Koopa Trooper is caught by surprise (i.e., jumped on the head) it will not have enough time to concentrate its magic and cause harm. This does not apply to spiked troopers, of course. Since they all posess this ability, the Koopa Troopers are immune to this death-by-touch.
Power stars are undoubtedly a hot commodity. And where are they most abundant? In the Mushroom Kingdom, of course. Particularly in the domain of Princess Toadstool. She is the only Mushroom citizen capable of harnessing the power stars in such a way that she can protect her people from Bowser's magic spells. However, she's not as adept at using them as Bowser is, otherwise she'd be able to protect herself and prevent power star raids!