its hard to tell,
if the question is "Do bettas get along with guppy" the answer is no,
while some people have succesfully kept both together it is not recommended.
the problem with guppies is that the are litterally a swimming target for any potentially aggressive fish due to their large fins,bright colours, and somewhat slow swimming.
bettas generally get along best with blander, short finned fish, prefferably faster and able to hold their own in case of an acceptionally aggressive betta
they have even been succfully kept with larger community/semi aggressive fish such as gourami and less aggressive sharks such as the rainbow shark.
i hope that helps, but don't always count on it being your betta,
in some places (depending on breeding) guppies can be quite flimsy.
if your guppies are slowly dying off, its a good idea to check your water and scan for disease/parasites or odd behavior (scratching, etc.)
it could be a warning that there is something lurking in your lank!
which could be why the guppies, being the weakest in your tank, are dying
GOOD LUCK!