Namaskaram. I am answering your question as far as I learnt from Swamy’s lectures.
Since we all live in this leelaviboothi and are trapped in samsara, we tend to commit mistakes and get disturbed by materialistic suffering even after performing saranagathi and thereby we acquire karma.
Perumal in His Rama avatara killed Subahu but let Mareecha go away. This is to represent that Perumal will burn all karma acquired before saranagathi and will erase karma gained after doing saranagathi just like water on a lotus leaf.
Even Mother Andal in Her fifth pasuram says ” poya pizhayum pugutharuvan nindranavum theeyinil dhoosagum seppelo rempavai “
Thus, Srimannarayanan will definitely protect and forgive us once we surrender to His lotus feet but we should not continue committing mistakes.