One thing I'll note is that EYEKKAPTEPHTON is used in opposition to TAΓAΘON (which, as Don has demonstrated in the past, can appropriately be expressed as "The Good", which is Pleasure), so in this case, "the terrible" or "bad", I think, is referring to the general feeling of Pain.
Thanks to you guys for pointing this out. The common discussion across the internet as "the terrible is easy to endure" is probably in my view the most damaging aspect of talking loosely about it.
The point of PD3 and PD4 from which these are derivations seems to me to be almost certainly, as you are stating, to be directed at "the good" or "what is good in life" and "the bad" or "what is bad in life," in a generic and philosophical way. Give the wide net that is included with Epicurus' view of "pleasure" then there's a corresponding wide net regarding "pain." The thrust of Epicurus' views on pleasure and pain are completely defensible and persuasive when put placed in its full philosophic context. In the form it's trumpeted widely, as for example in today's Wikipedia, I continue to see its use as an abomination.
No need to go through all this again but we continuously have new people, so might as well link to my prior thread here for those who have not seen it.