The 'most' would probably be 3. But it still isn't quite natural. What about: She saw him accidentally stabbed with a knife by the fugitive last night - that's more natural I think.

I have thought about this a little because of the disagreement between William and Gary. In my view, "by accident" and "accidentally" are interchangeable when the meaning is the opposite of "deliberate". When the meaning is "by chance", then you can only use "by accident". You also have to be careful about word placement. Consider the following sentences: 1. She saw him accidentally stabbed with a knife by the fugitive last night. [Fugitive did not intend to stab him.] 2. She saw him stabbed with a knife by the fugitive by accident last night. . [Fugitive did not intend to stab him] 3. By accident, she saw him stabbed with a knife by the fugitive by accident last night. [Either (1) She did not intend to see the event. Perhaps she was looking the other way, trying to avoid it, but she still saw it. Or (2) she arrived at the scene by chance. She wasn't trying to avoid the scene] 4. She accidentally saw him stabbed with a knife by the fugitive by accident last night. [She did not intend to see the event. Perhaps she was looking the other way, trying to avoid it but she still saw it.] 5. There was a struggle between a fugitive and another person last night, and by accident, she happened to witness it. [She arrived at the scene by chance. She wasn't trying to avoid the scene] My sentence 3 may be ambiguous in certain contexts and so I would recommend another option.