How come 将 is a "future marker" and a "passive marker"?
将 before a verb is functioning as a [modal verb]: 'will / will soon'

Example:
(将)回家 = (will) go home
(将)死 = (will) die/ will die (soon)

将/把 before a noun is functioning as a [depositive marker] that marks the noun to be deposited by the verb

Example:
打死他 (beat him to death)
将他打死 (take him, and beat to death) - 将 marks 他 as the object that's being deposited by the verb 打死

将 https://en.wiktionary.org/wiki/%E5%B0%87
Phono-semantic compound (形聲, OC *ʔsaŋ, *ʔsaŋs): phonetic 爿 (OC *braːn, *zaŋ, “bed; small table”) + semantic ⺼ (“meat”) + semantic 寸 (“hand”) – &gt;to offer meat as tribute by putting it on the table.

&quot;to offer meat&quot; is to &quot;deposit the meat' that's how 将 acquired the function of [depositive marker] that marks object to be deposited of
Just read the article at WiKi
Stumpy Joe Pete wrote:

how did the &quot;to want, to wish, to approach&quot; meaning come to be attached to that character? Did it come about later, or have both meanings been attached to the same character/pronunciation as far back as we can go?

from Sino-Tibetan root:
⺼ (“meat”) +  寸 (“hand”) is the semantic component of 將 . It describes the action of handling a piece of meat; 爿 (table) is a semantic + phonetic component of 將.
The original meaning of the character 將 meant &quot;to offer meat as a tribute to gods by putting it on the table&quot;
The action of 將 described (put meat on the table) acquired the function of [depositive marker] later
From Austroasiatic root:
&quot;Khmer ចង់ &quot;  (to want, to wish, to approach) was imported to Chinese language in ancient time, and the existing character 將 was loaned to express this word for phonetic  reason. Since  &quot;to want, to wish&quot; is related to future event. meaning of the modal verb &quot;will&quot; was added to 將 later