Airline rating systems are normally statistical in form. We all know that there are "lies, damn lies, and statistics."
Hence, if the ratings are from interpretation of statistics, different statistics can be used to paint almost any picture you want.
The ratings publicised should be examined with that in mind, and once you know from whose perspective you should be looking, you can make your own judgement.
However, just because an airline has not had an accident, it need not necessarily be "safe".
It may not have been flying very long, or it may not have flown a "statistically significant" number of passengers or sectors.
Probably the most practical method of evaluating safety is to find out how the particular airline is rated by insurance companies, they have a financial interest in being correct.