
A killer riding in the back of the taxicab upon shooting Paul Stine would have received some back spatter to his fingers. Even if he was wearing gloves, blood would have deposited onto the fingers of the gloves. Still holding the gun in say his right hand, he opens the right rear passenger door with his left hand (gloved or not) and exits the rear passenger door, pushing it shut with his left hand, arm, body or leg. The Zodiac Killer then puts the gun away and naturally opens the front right passenger door with his right hand, depositing blood from his hands or gloves onto the outside door handle. This is how blood got onto the outside front passenger door handle.

He now has to use the inner front passenger door handle to exit the taxicab, thereby depositing blood onto the handle. But there is no mention of blood being found on the inner handle. If he used a handkerchief over bare hands to avoid putting bloody fingerprints on the inner door handle (which he wouldn't have done if he was wearing gloves), then how would he have deposited bloody fingerprints to the outside door handle when supposedly wiping it down using a handkerchief over his fingers? In wiping away his fingerprints deposited when entering the front passenger door, we would have to believe he accidentally deposited bloody fingerprints in a wiping motion through a handkerchief as he departed.. It simply isn't realistic.
However, if he had deposited bloody prints on the front passenger door handle when exiting the rear of the taxicab and entering the front, it is possible that when he finally exited the taxicab for the final time, he attempted to wipe the outside front passenger door handle but inadvertently left a small print behind (or he didn't wipe it down at all). If gloves were used throughout (which is unlikely), there was never any need for a front passenger Zodiac Killer to touch the outside front door handle when he exited.. You would just push the door shut. Also, a back seat passenger entering the front of the taxicab would have no need to close the front passenger door when removing a section of Paul Stine's shirt, and therefore wouldn't need to touch the inner passenger door handle to leave the taxicab, which is why no traces of blood were mentioned as being present on the inner handle. There is a scenario for a back seat passenger to leave bloody prints on the outside front passenger door handle, whether he wiped it down later or not. For a front seat passenger, there were two options. [1] Leave the taxicab and simply push the door shut, thereby leaving no blood on the outer handle, or [2] Place a handkerchief over your fingers and hand and wipe the outer handle to remove the bloodless prints deposited when entering the taxicab. Either way, recognizable fingerprints deposited with a bloody hand is impossible in the first scenario, and extremely unlikely in the second.