Tip: If you give a Muncher tile custom graphics, it will still act like a coin when a silver P-switch is active, whether it looks like a coin or not. So make sure to use ExAnimation if there's a silver P-switch in the area.
HxD is a simple to use but very powerful hex editor. Some of the more notable features include: fast opening of large files, file comparison, tabbed interface, file sharing, and many more.
See the readme for details.
Note that making hex edits to a Super Mario World ROM for hacking purposes is faster and more easily done by creating hex edit patches using this quick tutorial.
The tool has nothing to do with the ROM crash. Either you did something wrong or the person gave you a wrong offset or even some patch already edited the spot, causing the area to corrupt.
The solution is simply undo that move, if you know what was the previous value. If you don't know what was the previous value, then restore your ROM to the previous state in a separate copy, open it, go to that offset, see that value, and copy back to the corrupt ROM.
HxD is one of the best all-around hex editors and it simply does what it's supposed to.