Given what the various dictionaries define I think "murder" is the more appropriate word, although there is some debate. So you can see for yourself I have posted some citations below.
From the Dictionary of Biblical Languages with Semantic Domains:
8357 רָצַח (rā∙ṣǎḥ): v.; ≡ Str 7523; TWOT 2208—LN 20.61–20.88 (qal)
murder, kill, i.e., take the life one another so as to cause a state
of death (Ex 20:13; Nu 35:6, 11, 12, 16,17,18,19, 21,25, 26, 27,28,
30,31; Dt 4:42; 5:17; 19:3, 4, 6; 22:26; Jos 20:3, 5, 6; 21:13, 21,
27, 32, 38; 1Ki 21:19; Job 24:14; Jer 7:9; Hos 4:2+), note: this
action can refer to an accident, manslaughter, premeditation, or
governmental execution; (nif) murdered, be killed (Jdg 20:4; Pr
22:13+); (piel) murder, kill (Ps 62:4[EB 3]; 94:6; Hos 6:9+), note:
for piel ptcp. as a noun in 2Ki 6:32; Isa 1:21, see 5344.5
From Strong's Enhanced Lexicon:
7523 רָצַח [ratsach /raw·tsakh/] v. A primitive root; TWOT 2208; GK
8357; 47 occurrences; AV translates as “slayer” 16 times, “murderer”
14 times, “kill” five times, “murder” three times, “slain” three
times, “manslayer” twice, “killing” once, “slayer + 310” once,
“slayeth” once, and “death” once. 1 to murder, slay, kill. 1A (Qal) to
murder, slay. 1A1 premeditated. 1A2 accidental. 1A3 as avenger. 1A4
slayer (intentional) (participle). 1B (Niphal) to be slain. 1C (Piel).
1C1 to murder, assassinate. 1C2 murderer, assassin
(participle)(subst). 1D (Pual) to be killed.
From The Abridged Brown-Driver-Briggs Hebrew-English Lexicon of the Old Testament:
vb. murder, slay — Qal murder, slay, with premeditation; c. acc.
pers., unawares; slay as avenger; esp. pt. as subst. = slayer,
manslayer, without intent; murderer, with intent. Niph. be slain;
murdered. Pi. (intens.) murder, assassinate; Pt. as subst. murderer,
Swanson, J. (1997). Dictionary of Biblical Languages with Semantic Domains : Hebrew (Old Testament) (electronic ed.). Oak Harbor: Logos Research Systems, Inc.
Whitaker, R., Brown, F., Driver, S. (. R., & Briggs, C. A. (. A. (1997). The Abridged Brown-Driver-Briggs Hebrew-English Lexicon of the Old Testament : From A Hebrew and English Lexicon of the Old Testament by Francis Brown, S.R. Driver and Charles Briggs, based on the lexicon of Wilhelm Gesenius. Oak Harbor WA: Logos Research Systems, Inc.
Strong, J. (1996). The exhaustive concordance of the Bible : Showing every word of the text of the common English version of the canonical books, and every occurrence of each word in regular order. (electronic ed.). Ontario: Woodside Bible Fellowship.