Is it bad to kill tasks in android, by using things such as holding down the home key and selectign "close all"? I use advanced task killer pro and also hold down the home key until a list of apps open shows up and choose "close all". Someone was telling me this is bad for the device. Is this true? How else can I speed up the device when it is running slowly?

I forgot to mention what I find strange is when I hold down the home key to display running apps, its list is different than the list of running apps advanced task killer has.
Short answer is: "That's Ok for device, don't worry"

Long answer is: killing task is the only way to speed up your system. When you kill an app by task killer, you close the app itself and all services or receivers bounded to it. That action would be harmful for the application, if it's downloading some data from the internet (some content update for example). Or application will stop sending notifications about new events. Almost all core apps will raise again in a few minutes. That's Ok for device, don't worry.