Well..... are you sure that's what it says?
Let's look at it:
Gen 4:14
הֵן֩ גֵּרַ֨שְׁתָּ אֹתִ֜י הַיּ֗וֹם מֵעַל֙ פְּנֵ֣י הָֽאֲדָמָ֔ה וּמִפָּנֶ֖יךָ אֶסָּתֵ֑ר וְהָיִ֜יתִי נָ֤ע וָנָד֙ בָּאָ֔רֶץ וְהָיָ֥ה כָל־מֹֽצְאִ֖י יַֽהַרְגֵֽנִי
The part to focus on is at the end. "
וְהָיָ֥ה כָל־מֹֽצְאִ֖י יַֽהַרְגֵֽנִי" It literally says "And it will be any find me will slay me". "
כָל" is any, not anyone, just plain any. Making it a little more readable, we can add a word "that". So the phrase is "... and it will be any [that] find me will slay me." That's it. A person isn't specificied. Cain is saying "any" not anyone.
Gen 4:15 It's basically the same thing as in the previous verse.
וַיֹּ֧אמֶר ל֣וֹ יְהֹוָ֗ה לָכֵן֙ כָּל־הֹרֵ֣ג קַ֔יִן שִׁבְעָתַ֖יִם יֻקָּ֑ם וַיָּ֨שֶׂם יְהֹוָ֤ה לְקַ֨יִן֙ א֔וֹת לְבִלְתִּ֥י הַכּֽוֹת־אֹת֖וֹ כָּל־מֹֽצְאֽוֹ
"
כָּל־הֹרֵ֣ג" - any [that] slay
"
כָּל־מֹֽצְאֽוֹ" - any [that] find him
Gen 4:16, Cain "knew" his wife and they conceived... It didn't say he found a wife. Cain could have brought a wife with him? We're not told that Adam and Eve had no other children. Yes it would have been incest, but that's not uncommon for primitive people. There is a legit theory, in my opinion, that Cain and Abel had twin sisters. Legit enough not to make assumptions. See below:
Aclima - Wikipedia.