I did a lot of image search on google and found out the original source of the image is http://www.ghadahaleidi.com/
because there is a lot of debate I posted the answer myself and here are the different sources of the answer. These article with answers have an oldest posted date, the articles with answer deep suggests are part of brain puzzle category and someone did it just for a prank (the article of gpuzzle do not have a post date) they are hiding the post date of article
Answer spoiler alert ( don't look)
The answer is 4 because of the scratch on his neck, the knife is missing and torn piece of his shirt is near the victim. You are assuming the red thing near the women is blood but it is not blood but the torn piece of the killer shirt which is also missing from the killer end if you see carefully.
As you can see in the image there is a woman on the left who got killed by one of the following suspect tags by number 1,2,3,4 you can see them on the right side of the image. justify your answer with proper statement will be accepted.
Hint 1: a waiter is bringing a glass of water but 3 & 4 is already having one.
Hint 2: Everyone is having at least 2 chairs with the table but only suspect 4 is having 1 chair?
Hint 4: 3 & 4 can be watched easily by 2 and nobody is watching 1 & 2.
Hint 5: the chair behind 2 seems like someone recently moved from there.
Hint 6: suspect 4 is not having a knife with him and his shirt tucked out but there is more.
Hint7: why there is no purse with the women who died?
Hint8: There is no table with extra food that means the women just came in and went to the washroom to wash her hands.
Hint9: Suspect 2 is having the best view he can watch everyone from that point. the chair behind him seems misplaced.
Hint10: it seems like there is a scratch on the neck of suspect 4
Hint11: we don't know anything about suspect 1 he is sitting idle not ordered any food yet. we can't see his table also nobody is watching on him.
Hint12: apart from suspects waiter can move freely from one place to another he can get a knife from any table easily. He can also access ladies washroom quietly we don't know where the door is for washroom