Hi there
First, it's not un-common that when you patch a ROM you'd get message from your emulator that the ROM is a bad ROM, seeing as it recognizes that the ROM was altered.
Now about the copies of the ROM per se, you need the "good" version of the ROM, the one usually marked simply as Slayers (J).smc . Sometimes you might find one with an (!) along with the title giving you Slayers(!)(j).smc. Both this & the one without an (!) are the good version of the ROM, any other versions are bad dumps of the game.
Now that you have a copy of the good ROM, you need to find out if your ROM has a header or not, there are two ways to find this out:
1. Compare your ROMs actual file size against that which the patch's readme file recommends, if they are not the same, your copy might be headered. This ROM by the way needs an non-headered ROM.
2. Google & download "Ucon64" & chances are the first entry on the search results page will lead you to ucon64's home page.
Download ucon64 ^ it's frontend u-fox (I recommend getting u-fox as it's GUI driven, unlike ucon64 itself which is purely command-line based.
Now once you have them start up u-fox then on the SNES tab click on "show info" then locate your ROM, it will then show you the information about your ROM.
Incidentally, patching ROMs often messes up the ROM's checksum values causing an error when the game is loaded up, in this case use "fix checksum" to correct the game's crc values so it can be properly loaded by an emulator. You might still get the "bad ROM" error even after you fixed the ROM's checksum, but that would only be because the emulator "sees" the changes the patch did to the game.
Now if this isn't the problem,I suggest you try getting a hold of the latest versions of either BSNES oe snes9x (version 1.52) as according to the "projects page" article on RHDN, "most" emulators do tend to act up with the Slayers ROM itself.
Hope this helps you out
If not wait for the replies of the awesome admins of this forum to help you in the more technical side of things