Apple wasn't suing HP. But whether it was HP or Apple that killed it, that's a tough call to make. Apple has made it exceedingly difficult for all tablet makers to get a foothold in the market, but at the same time, it was HP that set unrealistic goals for the Touchpad. They eventually sold at fire sale prices, but for those that received one, you can tell just by the quality of the packaging that HP had big, big plans for the Touchpad. You can see this from the advertising campaign as well.

As a device, I don't think the TP is a failure. In fact many people that use it say it's the best tablet out there. But whether it was Apple's dominance or HP's incompetence to set realistic goals that killed it... Tough call.