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In German, you don't use the plusquamperfect (hattest verlassen) without a context. That is to say, you use it only to make clear that the event happened before a different event that is also set in the past.

Du hattest mich verlassen, aber du bist zurückgekommen.

The perfect tense (hast verlassen) is the most common way to describe a past event in spoken German (the preterite tense is rarely used).

Du hast mich verlassen, ohne mir ein Wort zu sagen.

That sentence can be understood as a simple statement (of course it could have a very reproachful tone). In any case, it could stand on its own.

Du hattest mich verlassen, ohne mir ein Wort zu sagen.

In this sentence it is implied that something happened meanwhile (between the act of leaving and the act of uttering that sentence). Whatever it is that happened should be clear from the wider context or be mentioned before or after this sentence.

Du hast mich verlassen, ohne mir ein Wort zu sagen.

The verb form hast verlassen is Perfekt. This event happened in the past of the reality.

Du hattest mich verlassen, ohne mir ein Wort zu sagen.

The verb form hattest verlassen is Plusquamperfekt. This event happened in the past of the narration.

If you used the latter sentence, a listener expects you to continue with your narration, using Präteritum tense. If you used Präsens or Perfekt in the next sentence, that would mark the end of your narration.

Ich habe eingekauft. Jetzt mache ich das Essen.

This is telling what happened in the recent past and present.

Ich hatte eingekauft. Dann machte ich das Essen.

This is narration about a line of events (most likely in the past).

Ich hatte eingekauft. Jetzt mache ich das Essen.

This is narration, followed by telling what happens in the present. A listener would be puzzled, asking you what's the connection between your story and your current plan. The end of the story must be missing!