There seem to be two different negotiation tactics here.
You have the democrats, who had the position of being for what DHS wanted, 1.6 in increased funding, who approved and passed legislation to that effect. After the deal was agreed to, the deal changed on Trump’s side, with him now demanding 5.7 b for a wall. A very natural negotiation tactic in that scenario is to say, “Well, now you are getting nothing, as you can not hold to your end of the bargain that you made.”
Then you have Trump, who got the increased funding that was asked for by DHS, who then was told by right wing media that he looked weak if he took the deal. So, after he has agreed to it, he changes it. In order to have a position, rather than appeal to voters, or appeal to reason or logic, he instead appeals to holding 800,000 people, along with pretty substantial parts of the economy, hostage, to get what he, not the voters, not Law enforcement, not DHS, wants.
The first there is a fairly standard reaction to someone who has gone back on what they agreed to, and is the only way to deal with someone who will not deal in good faith.
The second is not honest, it is just bullying people to get your way, no matter who is harmed in the process.
I find the first to be entirely defensible, logical, reasonable, and moral.
I do find the latter to be, while not defensible, reasonable, or moral, entirely logical, however, as there is a reason why criminals take hostages in order to get their way. Then there are those who defend that tactic.