When I run these lines in my terminal on macOS I get the correct result
hex=$(echo -n 'betty' | xxd -p)
echo $hex
6265747479
but when I run them in a bash script I get something completely different
sh myscript.sh
62657474790a
It's like its put a carriage return on the end for some reason. WHY?
Different versions of echo do different things when given -n as their first argument. Some print it as part of their output, some interpret it as a flag, who knows what's going to happen. According to the POSIX standard for echo, "If the first operand is -n, or if any of the operands contain a backslash character, the results are implementation-defined."
The most reliable way to print a string without a linefeed after it is with printf. It's slightly more complicated because you have to give it a format string as well as the string you want printed:
hex=$(printf "%s" 'betty' | xxd -p)