Could dying of massive internal injuries/multiple traumatic injuries be painful?

Possibly. Fractures will usually cause the most pain, but if there is massive blood loss and shock the patient will become comatose. In that state, the brain shuts down and we don't think pain is experienced as when awake. So, in the wake state, yes it is painful; in a coma, not so much.

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Help please. Could dying of massive internal injuries/multiple traumatic injuries be painful?

Possibly. Persons experiencing massive or multiple internal injuries enough to cause death, will most likely be rendered unconscious, especially if the brain is involved, and therefore not experience pain.
For awhile, yes. This form of death may or may not happen quickly -- and in the interim there could be great pain but little ability to do anything about it. Eventually you might exsanguinate, but the length of time depends on the extent and type of injuries. Hoping you are not planning to try this. And if you are, please tell your doctor, your therapist, or a trusted friend. Get help for yourself.