Remove the repository from your sources.list
However, what you describe doesn't sound like how synaptic works.
Synaptic is used to download and install files from it's database. It is not used to install files you have already downloaded - dpkg is the tool for deb files - though I understand that it may be configured to do this.
Synaptic and apt follow instructions in the deb file as to how to resolve dependencies. This is interesting because the files sought in your case are all rpm files - a format synaptic does not handle well. It is unlikely that a deb file will have rpm dependencies.
So the question must be: how are you going about installing second life for linux? What is the file you downloaded, and where did you get it from? What are you actually doing to install from this file? (Command used, sequence of menu-clicks, whatever; be pedantic.)
The official SL Linux client is here
. And it arrives as a tarball. All you do is unpack it, cd into the top directory, and run ./secondlife.