I'll take a stab at this.
In the past tense, with 殺されるところだった vs 殺されかかった, I think they are mostly the same. One expect that those things didn't actually happen.
For the present tense, I'll quote from 大辞林
~かかる means that they are slightly before the level of ~ところだ. So they are very similar, but I think in general with ~ところだ there is a stronger nuance that one expects the verb to happen (assuming no any interventions).
For the case of 殺される, I think in the dictionary form of the verb, both can be used in the sense of being very close to dying/heavily injured. I'm not sure about the exact context of the following image, but one could easily imagine Goku being on the verge of death after being punched and injured.
However, 殺されかかる can be used to describe another type of situation:
Obviously if Goku were to get by one of those punches, he might end up in a situation similar to the first image, but Goku is a professional and has it completely under control. One doesn't necessarily expect any of those punches to land on Goku.