So by the logic of some US state if we would have took part in an armed robbery and accidentaly killed a man he may be sentenced to death but infecting 300 people with a potentially lethal disease gets him....15 years
No, it can be life in that case. There's evidently a distinction in MO law between exposing someone to HIV and infecting them. From OP's article:
Malicious intent would imply he intended to harm them. Obviously he didn't - he probably hoped they wouldn't get hiv...
Not necessarily disagreeing with you here, but
that's not always the case... 
