Why are words ending in -icious evil

I think this is pretty much a coincidence.

The Latin suffix -osus approximately means "full of" (or -ful). There are a lot of -ful words in English that have negative meanings, such as painful, sinful, scornful, disdainful, mournful. But the negativity mainly comes from the meaning of the root, not the suffix.

In the case of vicious, malicious, pernicious, avaricious, suspicious, the consonants before -ious are not actually all the same in the Latin sources. They are from

• vitium
• malitia
• pernicies
• avaritia
• suspicio/suspitio

The -it- in avaritia and malitia occurs in a common abstract noun ending in Latin: -itia. The other words just happen to be built on stems containing -it- or -ic-. Latin t and c both came to be pronounced the same way in this context in the varieties of Latin and French that English got the -ious adjectives from, but they were originally different consonants (although suspicio already showed variability in spelling in Latin).

As the comments mentioned, there are other positive -icious words than delicious such as auspicious, judicious.