Whoever Kills a Believer…

وَمَن قَتَلَ مُؤْمِنًا خَطَأً

wa man QaTaLa mu’minan khata’an..
and whoever KillED a believer by mistake… [al Nisa 4:92]

وَمَن يَقْتُلْ مُؤْمِنًا مُّتَعَمِّدًا

wa man yaQtul mu’minan muta’ammidan…
whoever KillS a believer on purpose…[al Nisa 4:93]
Past tense implies:the event occuring Once [in the Past].
Present-Future tense implies: Continuity [it may happen again in the Future].
The one who does it by mistake will not do it again, but the one who kills intentionally, may repeatedly do it.
This is why – the one who does it purposely should be punished (i.e. a life for a life.) The one who does it unintentionally – he can pay blood money.