Suitcase theory, and the Death of Vincent.
It would make sense if inside the suitcase was Marsellus's soul. Hence Ezekiel 1:1 - 1:25. More specifically Ezekiels 1:16 and 1:20 -
Ezekiel 1:16 The appearance of the wheels and their work was like unto the colour of a beryl: and they four had one likeness: and their appearance and their work was as it were a wheel in the middle of a wheel.
Ezekiel 1:20 Whithersoever the spirit was to go, they went, thither was their spirit to go; and the wheels were lifted up over against them: for the spirit of the living creature was in the wheels.
Meaning that the living creature's souls were inside of different objects. In Marsellus's case, the suitcase. Without the suitcase, Marsellus was ruthless i.e. throwing Tony Rocky Horror out of the building.
I also agree with the theory that Vincent died because of his lack of belief in divine intervention. Hence these passages:
Ezekiel 3:18 When I say unto the wicked, Thou shalt surely die; and thou givest him not warning, nor speakest to warn the wicked from his wicked way, to save his life; the same wicked man shall die in his iniquity; but his blood will I require at thine hand.
Ezekiel 3:19 Yet if thou warn the wicked, and he turn not from his wickedness, nor from his wicked way, he shall die in his iniquity; but thou hast delivered thy soul.
Meaning, Jules was a messenger sent to warn Vincent that if he didn't stop killing, he would die.