Noxious
Dangerous
The Latin root word "noxa" means harmful or dangerous. It is often used in English words related to injury or harm, such as noxious or obnoxious.
dangerous, Pernicious, malicious
The word "noxious" is nearly identical to the Latin noxius (harmful) from nocere (to harm, injure).(This same root produces the antithetical "innocent", i.e not causing injury.)
calamitous
"More harmful" and "most harmful." We would not use the -er and -est suffixes on the -ful which is already a suffix.
Dangerous in Arabic means "khatir" خطير
It means danger or dangerous.
Missionary
harmful, hateful or heinous
The opposite of harmful would be helpful. You could also say beneficial or fortunate.
Insidious means proceeding in a gradual, subtle way but with harmful effects. It often refers to something harmful or dangerous that is not immediately obvious or easily detected.