If all Joseph Smith needed was a stone and his hat to write the Book of Mormon, why the need for gold plates for which a murder was committed?
I presume the “murder” to which you are referring is Nephi’s killing of Laban. This killing was a part of Nephi’s efforts to obtain the brass plates, not the gold plates; and the worth of the brass plates to Lehi’s family and to his descendants (as well as the Mulekites) is well attested to in scriptures (see, e.g., Omni 1:14, Mosiah 1:3-4, Alma 37:1-10).
As to the question of why the actual plates were necessary if the seer stone (or the Urim and Thummim) rendered their presence unnecessary? For one thing, the plates provided concrete evidence for eleven official witnesses–and a handful of unofficial ones–that the Book of Mormon recounted the stories of real people, places, and events. For the world at large, of course, the plates obviously would never be made available as empirical proof of Joseph Smith’s claims–but the supporting testimony of multiple witnesses also made Smith’s claims especially worthy of extraordinary consideration in a world rife with, and generally skeptical of, individuals who claimed uncorroborated experiences with the Divine.
It may also be that the existence of gold plates sparked extra interest and attention in a region of upstate New York where treasure-seeking and “money digging” was already an established (and, among the educated classes, controversial) practice.
And finally, the efforts Joseph Smith had to make to actually obtain the plates–as well as his later struggles to safeguard them–were an opportunity for him to fine-tune his ability to obey, to listen to the Holy Spirit, and ultimately become the kind of prophet that God needed him to be.