What was Yehonason's assurance to David? Sha'ul was openly seeking to kill David!
Radak: He was sure that he would not kill David, after he swore not to (19:6). Even though after this, he tried to kill him via (a) a spear, (b) messengers to David's house and (c) pursuing him to Nayos, this was due to Ru'ach Ra, which came only at times (16:23). 1 David can guard himself at those times. Even though he would play for him at those times, David could avoid standing opposite him. Really, Sha'ul wanted to kill him even not at the time of Ru'ach Ra, but he did not show this, due to his Shevu'ah to Yehonason, or lest Yehonason be upset. Sha'ul's servants told David; Sha'ul revealed his intent to them, but hid it from Yehonason. This is why David swore "ch'Pesha Beini u'Vein ha'Maves" (verse 3).
Malbim: Yehonason thought that he sent messengers only to bring David to him lest he flee, but he would not kill him without telling Yehonason.
If this were true, Yehonason could have asked Sha'ul to tell all his servants to ignore any command to harm David, for surely it is due to Ru'ach Ra! (PF)
Why did he say "Ein Zos"?
Malbim: He did not send messengers to guard you in order to kill you - just the contrary, to guard you lest you flee him!
