"And that assumption of yours which you assumed about your Lord has brought you to ruin," meaning: destroyed you, "so you have become," meaning: you have turned into, "among the losers."
﴿وَذَلِكُمْ ظَنُّكُمُ الَّذِي ظَنَنْتُمْ بِرَبِّكُمْ أرداكم﴾ أهلككم ﴿فأصبحتم﴾ يَعْنِي: فصرتم ﴿مِنَ الْخَاسِرِينَ﴾.