It's been a while since I read the novel but was it always Dracula's plan to kill Harker or did he intend for him to leave at some point? I know Harker suspected it after Dracula forced him to write the letters but it didn't seem like he meant him any harm before he started snooping in the castle.
There's no indication that Dracula ever expected Harker to leave Transylvania. Harker realizes quite quickly that the Count has made him a prisoner, well before he's given the Count any reason to suspect him of knowing too much (emphasis mine):
When I found that I was a prisoner a sort of wild feeling came over me. I rushed up and down the stairs, trying every door and peering out of every window I could find; but after a little the conviction of my helplessness overpowered all other feelings. When I look back after a few hours I think I must have been mad for the time, for I behaved much as a rat does in a trap. When, however, the conviction had come to me that I was helpless I sat down quietly — as quietly as I have ever done anything in my life — and began to think over what was best to be done. I am thinking still, and as yet have come to no definite conclusion. Of one thing only am I certain; that it is no use making my ideas known to the Count. He knows well that I am imprisoned; and as he has done it himself, and has doubtless his own motives for it, he would only deceive me if I trusted him fully with the facts.
Dracula Chapter 3
As I say, this is some point before Harker first witnesses anything strange in Dracula's castle. Dracula extends Harker's stay, and forces him to write his first letters home shortly after this, but still well before he goes snooping around the castle:
"Have you written since your first letter to our friend Mr. Peter Hawkins, or to any other?" It was with some bitterness in my heart that I answered that I had not, that as yet I had not seen any opportunity of sending letters to anybody.
"Then write now, my young friend," he said, laying a heavy hand on my shoulder: "write to our friend and to any other; and say, if it will please you, that you shall stay with me until a month from now."
"I pray you, my good young friend, that you will not discourse of things other than business in your letters. It will doubtless please your friends to know that you are well, and that you look forward to getting home to them. Is it not so?" As he spoke he handed me three sheets of note-paper and three envelopes. They were all of the thinnest foreign post, and looking at them, then at him, and noticing his quiet smile, with the sharp, canine teeth lying over the red underlip, I understood as well as if he had spoken that I should be careful what I wrote, for he would be able to read it.
Dracula Chapter 3
It's not until later that night that Harker first witnesses Dracula clambering down the castle wall, and another four days before he first encounters Dracula's brides.
The format of the novel precludes us from knowing too much of Dracula's mind, so we can only speculate as to his intentions, but his actions certainly don't suggest that he's ever planning on letting Harker return home, regardless of what he does or doesn't know.