I wanted to use the Kompare utility, that's all. My Ubuntu is otherwise using the ubuntu default Unity desktop framework. Have I just destabilized my desktop?
I wouldn't think so. This is common with GUI components from specific DE's. Generally they can be run without the DE itself (within a different DE, or whatever), but they require shared components which have been compiled by the distro with other dependencies, etc. It can even happen with non-GUI components that aren't part of a DE: the worst example I've seen lately was installing doxygen (a documenter for source code) on a minimal debian system: it required 1/2 GB of dependencies, because doxygen uses latex which requires GUI stuff (and note the system already had X installed!) up to and including some Qt and KDE parts, I think.
It seems like a lot, but presuming you don't have a lot of restrictions on disk space, it should not make any other difference to your system. Most of that stuff will never even get loaded into memory; shared objects can be partially loaded, but the dependency system assumes everything might be needed. This is a actually a very reasonable strategy. Keep in mind that most linux distros, including ubuntu, have a relatively small footprint when first installed; I think it is generally <20 GB. Compare that to other mainstream OS's that will plunk at least 100 GB down from the start.
I seem to have installed KDE on Ubuntu 13.04 desktop
Do you mean that KDE has taken over as your default desktop? If so, then you should ask a question (or search around) about configuring the default desktop on ubuntu.
If not, then don't worry about it. It is not wrong and should not be a problem. It is common to have multiple DE's installed on the same system and in many cases it is even possible to run several different DE's at the same time with separate logins sessions.