All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Telling lies is to intentionally say or write something which is not true. It is one of the grave major sins. It will inevitably doom the liar to Hell. May Allaah The Almighty Save us from Hell. It was narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “Lying leads to wickedness, and wickedness leads to Hell, and a man may keep on telling lies till he is counted in the Sight of Allaah as a liar.” [Al-Bukhari and Muslim]
Imagining death in the way that you have mentioned is not from telling lies at all for you do not tell something that can be true or not. Rather, this way of imagination can be good if the individual wants to get a reminder and admonishment from it, or increase his fear of Allaah The Almighty, or encourage himself to practice more righteous deeds and keep away from doing evil deeds. However, if you did such imagination aimlessly, then this would not be good for you for you will be spending your time to no avail.
A wise person should make the best use of his lifetime and spend it in what can be beneficial for him in this world and the Hereafter for he will be held accountable for whatever he does in his lifetime.
Allaah Knows best.